Module 1: Occupational Hygiene Principles

Module 1: Occupational Hygiene Principles



Hello. Welcome to "Occupational Hygiene
Principles". My name is Pete Raynor. I’m an Associate Professor at the University of
Minnesota School of Public Health. The learning objectives for this module are that,
by the end, learners should be able to classify
the types of hazards that workers face, define exposure and related terms, list the
routes by which workers can be exposed to
hazardous agents, and describe the occupational hygiene
framework of anticipating, recognizing,
evaluating, and controlling workplace hazards. As we begin to discuss the occupational
hygiene framework, I should make the point that
"occupational hygiene" is a term used interchangeably with "industrial hygiene".
"Occupational hygiene" is used more frequently
in Europe and other parts of the world whereas "industrial hygiene" is used more
commonly in the United States. I'm primarily going to use "occupational hygiene"
in this module because I think it describes the
profession more accurately. The American Board of Industrial Hygiene
defines industrial hygiene as "the science of
protecting and enhancing the health and safety of people at work and in their communities”.
This definition makes sense from the standpoint
of protecting people at work. However, it also includes another critical aspect
of occupational hygiene: the protection of people in the community who
may be affected by what others do at work. Goelzer defines occupational hygiene as "the
science of the anticipation, recognition,
evaluation,and control of hazards arising in or from the workplace and
which could impair the health and well-being of
workers, also taking into account the possible impact on
the surrounding communities and the general
environment." This second definition of occupational or
industrial hygiene is more comprehensive. I also like that it includes the occupational
hygiene framework of anticipating, recognizing,
evaluating, and controlling hazards. We need to be able to understand when
potential hazards may be present, notice them
when they are there, know how to determine if they are a problem,
and then do something about them if they are a
problem. With whom do occupational or industrial
hygienists interact? They interact with people on the job. Most
important are the workers. However, this also includes the owners,
managers, and supervisors of different
workplaces without whose support we can get little accomplished, regulators like those that
work for the Occupational Safety and Health
Administration or OSHA at either the federal or state level, and members
of the public who may be affected by what goes
on at the worksite. Occupational hygienists interact with their
colleagues in other health and safety fields
including occupational physicians, occupational health nurses, safety and
environmental specialists, occupational
epidemiologists, and occupational hygiene technicians who may
carry out some of the measurements and
sampling that are designated by hygienists. Finally, occupational hygienists also interact
with engineers and facilities and maintenance
personnel. Engineers may be tasked with
carrying out changes in workplaces to make
them more healthy or safe, and facilities and maintenance personnel can
affect workplace health and safety by the regular
maintenance they conduct, the cleaning that they do, and other changes
that they may make to work
environments. Let’s talk next about anticipating and
recognizing hazards. The way I'm going to do that, at least initially, is
to talk about my high school job I worked at a grocery store in Irondequoit, New
York, a suburb of Rochester. I worked there from late in my junior year of high
school through the summer after my sophomore
year in college. When I went to work at the grocery store, my
first job there was to collect carts outside. Today, many cart pushers have automated,
battery-powered pushers to move trains of carts. Back when I did the job, I worked much like the
person in this image: we assembled long trains
of carts and pushed them across the parking lot. So, there was a lot of a heavy pushing, and
there was also a chance to get your fingers
pinched between the carts. It was hard, tiring work. We would sometimes
make very long trains of carts on purpose to see
how long we could make the train and still push it back to the front of the lot. Sometimes,
the long trains got a little out of control and hard
to stop because they had so much momentum. We had to be concerned about vehicles in the
parking lot. Often, the parking lot was
congested, particularly at busy times around holidays or on weekends. Although I was never
injured by an incident with a car, there were
many close calls and I had to watch myself around cars that were moving too fast or were
having difficulty backing out of a parking space. During different times of the year, we were
exposed to either heat or to cold. Because
Rochester, New York is on Lake Ontario, we had a lot of lake effect snow and frequently it
would be slippery in the parking lot during the
winter in addition to it being cold. In the summertime, it could be very hot and we
would feel it after four hours of pushing carts
around. We would sometimes be faced with shoplifters
when we worked outside the store. On one particular occasion, I was collecting
carts on a sunny afternoon. I was out there by myself, it wasn’t very busy,
and I was off in my own little world as I was
picking up carts. So, I wasn’t really paying too much attention to
what was going on other than in my own
immediate vicinity. Suddenly, I saw this guy running toward me, and
as he got closer, he yelled to me, "Don't do it,
man! Don't do it!" I'm thinking to myself, "Don't do what?" The guy ran past, and I was
still trying to understand what he meant, when
my supervisor runs up to me and says, "Pete, why didn’t you stop him?" I thought to
myself, "Stop what?" I had no idea what was
going on. It turned out that he was a shoplifter. So, there was a risk that, had I realized what
was going on and tried to intervene, I could've
been subjected to some violence. My supervisor was a little disgusted with me for
not divining that I should have stopped the guy. This particular supervisor didn’t like me very
much, and it was sometimes stressful to deal
with him. Eventually, I was given the opportunity to move
inside, at least most of the time, and be a
checkout clerk, which involved scanning a lot of items, putting them into bags, and lifting the
bags up and over a shelf to put them into the
customers’ shopping carts. I would also have to enter numbers into the
keypad on the register. Shifts on the registers
could last as long as eight hours with a lunch and two short breaks. Standing in one
place and repeating these same actions over
and over was tiring. One of the other things I would sometimes do
when I worked inside was to go to the back room of the store and bring sacks of paper
bags – 500 per sack – up to the front end. We didn’t have plastic bags back then, so these
were thick paper bags. I would take a large cart to the back room, go
into the truck trailer where the sacks were
stored, climb a pile of sacks, toss about 15 sacks from
the pile onto the bed of the trailer, pick the sacks up and pile them onto the cart,
and push the heavy cart through the store to the
front. Once there, I would have to unload sacks by
each register, open the sacks with a box cutter,
and stack the bags in the storage area below the register where the clerks could pull
them out and use them for customers.
It was a pretty tough job. When I worked inside at the front end, there was
always a chance that we would have to deal with
those shoplifters. One time, a couple of us chased after some
shoplifters who were trying to steal beer late at
night. We got out into the parking lot and found that
we were a little outnumbered because the
shoplifters were part of a larger group. Fortunately, the shoplifters and their friends
decided to leave the beer on the ground and
drive away rather than forcing a confrontation. Another time, several of us were working up
front, and a car pulled up outside the front
window of the store right near us. A bunch of guys piled out of the vehicle after
popping the hood, and we could see that there
was a fire in the engine compartment. We were, naturally, concerned about this, as
you might expect. After a frantic few seconds
discussing what we should do, I ran out of the store with our fire extinguisher
and put out the fire, trying to keep as far away
as I could. We were surprised about 10 minutes later when
the guys piled back into the car and drove away. I'm not sure how far they got; it was a topic of
discussion for my co-workers and me for the
rest of the evening. Among some of the other duties I had was
occasional maintenance work, especially on
weekends. There was one time on the Monday of Labor Day
weekend that I had to clean both the men's and
women's bathrooms for the store. It was clear that they had not been cleaned
since the previous Friday, and it was a pretty
eye-opening experience to have to clean those restrooms after that
amount of time. In short, it was not a fun job. There was lots of nasty stuff in the bathrooms,
and a variety of cleaning products needed to be
used. Another one of my occasional tasks was to take
materials to the back room to be disposed of. Large groups of fluorescent bulbs are changed
out at the same time in stores to make the task
easier logistically. This was the early 1980s: the fluorescent light
bulbs were not recycled at that time. When I was asked to dispose of the bulbs that
had been changed out, I would take barrels full
of them back to the trash compactor in the back room, pile them into the trash compactor,
close the door to the compactor room, press the
button to turn the compactor on, and we would then hear the bulbs shatter in the
compactor. As a teenager, this was a pretty
cool thing to hear all the crashing and smashing and eventually, after the compactor had stopped,
we would open the door, and we could see an almost magical haze of
shiny glass particles floating in the compactor
room. It was awesome to look at, man! Probably the most fun I had on the job was on
one full-day shift on the Saturday after
Thanksgiving when my co-worker Todd and I had the opportunity to hang holiday decorations
for the entire day. We climbed ladders and
reached out to hang things across the ceiling, We climbed ladders outdoors and hung garlands
and decorations across the front of the store.
There was a lot of climbing up and down and reaching this way and that, but it was a really
fun day because it was an unusual task to get
paid to do. We enjoyed it quite a bit. By this point, you may be wondering, “Why is
this guy droning on about his high school job?” Well, let’s think about my job and about the
potential hazards that I faced on the job. There were hazards that could have caused
unintentional injuries: fires like from the car,
vehicles in the parking lot, slips in the parking lot when I pushed carts, falls
from ladders when I hung holiday decorations,
sharp objects like box cutters, and pinch points like when the carts come
together and you pinch your fingers as you try to
line them up. In addition to the unintentional injuries, there
was a risk of intentional injuries. Violence was
a risk particularly when I faced shoplifters. Repetitive motion injuries were quite possible. I
faced the risk of an injury to my back from
pushing carts, lifting sacks of bags in the back room, and lifting
full bags into customers' carts at checkout. Wrist injuries were possible from continually
scanning items at the checkouts for long
periods of time. Temperature extremes when I worked outdoors,
both when it was hot and cold, could have led to
heat or cold strain. I was exposed to germs at work. Because I
was dealing with members of the public, I could've been exposed to their germs as they
sneezed or coughed near me and when I
handled their money. There were also the times when I cleaned the
public restrooms when there was a potential for
exposure to germs. Chemical exposures from cleaning chemicals
and the mercury in the fluorescent bulbs could
have been a health concern, too. Even stress could have been a concern. I knew
that my supervisor didn't like me very much. If I had cared more about the job than I did, I
may have felt stress that could have impacted
me negatively. There was a whole range of different hazards
that I was potentially exposed to in my
workplace. Although you may have been able to anticipate
that grocery workers face workplace hazards,
most of you, unless you've worked a very similar job, would not have been able to
recognize all of these different hazards. This is an important point because when you are
trying to anticipate and recognize hazards, you really need to get to know the job before
you can be effective at analyzing the hazards. Ultimately, workers are the experts on their own
jobs. If you are trying to understand where there is a potential for hazardous exposures to
whatever sort of agent you're concerned about,
you need to talk to the workers. If we generalize and categorize some of the
hazards that exist in different workplaces, we can anticipate and recognize chemical
hazards that include airborne particles such as
nanoparticles. Workplaces may contain different gases and
vapors, and especially solvent vapors as many,
many solvents are used in industrial settings. Heavy metals may be present, including molten
metals, metals used in electronics production,
and metals released to the air during machining. A large variety of skin irritants may be present
as well, with dermatitis being one of the most
common workplace diseases. There are physical hazards, hazards that affect
the senses or the whole body. These include noise, ionizing and non-ionizing
radiation, and hot and cold temperature
extremes. We can anticipate and recognize biological
hazards, including infectious disease agents, which can be of particular concern in the
healthcare industry, and mold. Agriculture workers may face mold in outdoor
environments or in barns, and construction workers may be exposed to
mold during renovations. Offices that have experienced water damage
may see mold growth in walls, ceilings, and
carpeting. There are injury hazards. Unintentional
traumatic injuries can occur. These include vehicle crashes, which are one of
the most common causes of fatalities on the
job. Violence, either among co-workers or involving
both workers and people from outside the
workplace, is an important occupational hazard. In addition, poor ergonomic conditions, including
repetitive motion, awkward posture, and heavy
lifting, may lead to musculoskeletal disorders. Occupational hygienists may be able to
anticipate, but find it hard to recognize, social and behavioral hazards like stress, sleep
deprivation, and substance abuse. These hazards can make it difficult for a worker
to perform her or his job safely and in a healthy manner, in addition to being risk
factors on their own. and in a healthy manner, in addition to being risk
factors on their own. Let's move on and talk about evaluating hazards.
Why would we want to evaluate hazards? I list
six purposes here. Let's move on and talk about evaluating hazards.
Why would we want to evaluate hazards? I list
six purposes here. First, we might evaluate hazards for compliance
purposes. The goal is to compares workers'
exposures to an exposure limit or a standard. For example, we can compare sound levels in a
metal stamping operation to the Occupational Safety and Health Administration's
permissible exposure limit for noise. We might try to measure levels of a hazardous
agent throughout a work environment with a goal
of identifying the source or sources of the agent. An example would be to create a concentration
map, almost like a topographical map, of a machining facility to identify the sources for
emissions of oil mist. In emergency situations, we might seek to
detect hazards that are immediately dangerous
to life and health. An example is the need to monitor hydrogen
sulfide levels when workers enter a manure pit to
perform maintenance or cleaning. Control measures might need to be evaluated.
The goal in this case would be to ensure that
interventions designed to reduce hazardous exposures are working as planned. An example
of this is a series of measurements performed to
ensure that airborne particles containing mouse urine proteins are kept within ventilated
enclosures during the change out of research
animal cages in university settings. We might also evaluate hazards as part of
research. The goal will depend on the hypothesis that is
being investigated, which is sometimes part of a
larger occupational epidemiology study. An example of this that I worked on was when
we measured silica dust concentrations as part
of an epidemiological study to determine the effect of the dust on the lung
health of taconite ore miners on the Iron Range
in northern Minnesota. Finally, we might evaluate hazards for risk
assessment purposes. In a sense, all of these other purposes are forms
of informal risk assessments. Here, however,
I'm talking about a formal risk assessment where the goal is to calculate exposure and/or
dose for a worker exposed to an agent of
concern, so that we might compare that exposure or dose to the potential health
effects from the dose in order to characterize the
risk of an adverse health outcome. An example is to measure radon concentrations
in building subbasements to estimate
cumulative doses that workers receive. We will talk more about risk assessment and
risk characterization in the next module. How do we go about evaluating hazards? One
way is by measuring them. We may measure a
hazard to detect it, just to see if it is there or not or we may want to know its concentration in a
medium like food, water, or, especially in
occupational settings, air. In addition to measuring agents in the
environment, we can measure things called
biomarkers within exposed people. In our context, biomarkers are substances
measured in some part of the body that indicate
the presence of an agent in the body. A biomarker may include a chemical of concern
or its metabolite, or some other biologic
indicator of exposure. This "biomonitoring" can be performed on
samples of urine or blood or you can measure
substances in tissues or hair samples. We can attempt to evaluate hazards using
modeling. We are not able measure everything
everywhere at all times. One way to get around these limitations is to
use mathematical models to estimate
exposures. The models can be used to predict concentrations or other relevant measures of
exposure as a function, ideally, of both time and
location. Ultimately, we will compare these
measurements or modeling predictions to some
sort of occupational exposure limit. These occupational exposure limits are
developed through the risk assessment process, which, as I mentioned previously, will be
discussed in a future module. In short, we relate health risk information from
toxicological and epidemiological studies to
exposure or dose data, decide what is an acceptable risk, and set an
exposure limit accordingly. When making measurements after performing
the risk assessment, we can compare our findings to the exposure
limit to determine whether the workplace may be
unduly harmful to people. We can start to discuss controlling hazards by
taking a look at some relevant definitions. The vocabulary used may vary depending on
one's perspective. We can think about "managing" hazards.
Merriam-Webster defines "manage" as "to work
upon, or try to alter for a purpose". The term "limit" is defined as "to curtail or
reduce in quantity or extent". To "intervene" is "to come in or between by way
of hindrance or modification". Finally, to "control" is "to reduce the incidence
or severity of, especially to innocuous levels". In many cases, these four words are used
interchangeably when talking about ways to
reduce exposures. The default word is often "control", but "control"
doesn't appeal to some experts because it
implies that you are always able to make the changes that you would like to in order to
reduce exposures to hazards. The term also
lends itself better to technological approaches to reducing hazard levels, whereas words like
"manage", "limit", and "intervene" seem to leave
open a broader array of approaches. In the way that I use the term "control", I intend
to leave open a wide variety of means for reducing exposures, not only
technological options. We are going to use the word "control" going
forward in this module, but keep in mind that the
other words convey equally valuable concepts. We refer to a "hierarchy of control" when
discussing approaches to reducing hazard
levels. The hierarchy goes from most preferred at the
top to least preferred at the bottom. At the top
is elimination of the hazardous agent. Can you just completely get the hazard or the
process that generates it out of the workplace
so that it's not an issue anymore? That option is rarely viable because it would
involve removing a process or a product that is
essential at that place of work. Next on the hierarchy are engineering controls,
which are physical, chemical, or biological
changes made to a process or a product that reduce exposures to the hazard.
Third on the hierarchy are work practice and
administrative controls, which are changes in how, when, or by whom tasks are performed in
order to reduce exposures. At the bottom is personal protective equipment,
or PPE, devices and garments worn by workers
to protect themselves from injury or illness. Let's talk a little more about why these
approaches are placed in the hierarchy in the
order that they are. Elimination is at the top because it is
completely effective for all workers, and because the responsibility for change is not
placed on the exposed person. Engineering controls are second on the
hierarchy because, even though they don't
completely eliminate the hazard, measures are put in place that should reduce
exposures for everyone, and the responsibility is not placed on individual
workers to reduce their own exposures. Engineering control includes a variety of
concepts such as substituting one type of
material for another in a process or a product, using automation so workers do not
need to be as close to a hazard, isolating the
person from the process generating the hazard or isolating the process from the person,
drawing contaminated air from a process away
from workers using ventilation, and installing control equipment such as an air
filtration unit to separate a hazardous agent from
the medium in which it is embedded. Work practice and administrative controls are
lower on the hierarchy because both management and exposed
workers are responsible for making changes. Therefore, we are starting to rely on people to
always perform their work in a certain way or at
a certain time, which may be difficult to achieve. Finally, personal protective equipment is the
least preferred approach when other options are feasible because individual workers must
use the PPE correctly each and every time they perform the task that creates the hazardous
exposure in order to be sure that they are not
exposed to the hazard. Ventilation is an important concept that we
should spend a little extra time talking about. In
particular, I will focus on local exhaust ventilation systems, which are intended to draw
contaminated air away from close to the point of
generation before workers can be exposed. There are many different types of local exhaust
ventilation. These types include exterior hoods. Both
images in the upper left show a flanged opening
connected to a flexible duct. A fan or blower downstream draws contaminated
air into the opening during the cleaning of a chamber in which nanoparticles
have been produced. Because the opening, in essence, needs to
reach out and bring in the contaminated air, it is
an exterior hood. Another type of exterior is shown in the other
image. This is a slotted hood where air is drawn
through slot openings, taking away particles and vapors that are
produced in front of the slots. This is an exterior hood because the hazardous
pollutants are outside the hood and must be
drawn into it. Another category of local exhaust ventilation is
partial enclosures. An example of a partial enclosure is the
laboratory hood on the lower left, in which any
process generating pollutants is enclosed on at least five sides and the pollutants are
drawn away with the air flowing into the hood. This air enters the hood through the sixth side
where workers can access the process. On the right is a drawing of a bagging process.
There is a clamp over the opening to the bag
from which, ideally, little dust will escape. To ensure that any dust that does escape does
not present an inhalation exposure risk to
workers, there is a partial enclosure around the bag filling
area that is ventilated and will draw away any
dust that escapes through the clamp. Partial enclosures combine the concepts of
isolation and ventilation, but they are not
complete enclosures. A ventilated glove box can be thought as virtually
a complete enclosure. Workers can put
containers of potentially toxic materials inside the glove box, use the gloves to open the
containers, manipulate the materials with any releases into the air being drawn away by the
ventilation, close the containers again, and then
remove the containers safely from the glove box. Let's consider work practice and administrative
controls next. Work practice controls alter how
workers perform a task. Ways in which someone can do work differently
might include scooping powders rather than
pouring them from containers in order to reduce airborne dust exposures, regular maintenance of
equipment, regular cleaning of work surfaces,
using wet cleaning instead of dry methods so that not as much dust is produced, washing
hands properly to prevent exposures to agents
on hands when workers eat or go home, continuing education and training on how to
work safely and in a healthy manner, and
emergency drills so that workers know how to exit their workplace while making sure to
shut down critical processes that pose risks to
responders on the way out. Administrative controls are measures that
change when and by whom work processes are
conducted. Examples include restricting access to areas
with potential hazards so that fewer people will
be exposed, the use of hot, warm, and cold zones during the
response to a hazardous materials spill where
only a few people with high levels of personal protective equipment are allowed into
the hot zone, security procedures to ensure that only people who are supposed to be at a work
site are present, limiting work time to reduce
mistakes due to sleep deprivation, and scheduling potentially hazardous work
operations during shifts when fewer workers are
present. On its web site, OSHA states the following
about personal protective equipment: "When exposure to hazards cannot be
engineered completely out of normal operations
or maintenance work, and when safe work practices and other forms of
administrative controls cannot provide sufficient
additional protection, a supplementary method of control is the use of protective clothing or
equipment. This is collectively called personal
protective equipment or PPE." The kinds of PPE that people wear to protect
various parts of the body range from hard hats
and safety glasses to hearing protection, respirators, gloves, safety shoes, and protective
clothing. There are many different types of PPE
within each of these classifications. The different kinds of protection need to work
together and they can be very effective, but they
require workers to use them properly. A worker wearing personal protective equipment
must be attentive every single time she puts it
on and takes it off. Otherwise, the PPE may not protect effectively
against harmful exposures. It’s challenging to be consistent with the use of
PPE so that the protection can also be
consistent. Let’s introduce some general concepts
regarding exposure and dose. Exposure can be defined generically as "the
intensity of the agent in question, time-averaged
in some way relevant to the adverse health outcome, at an appropriate
interface between the environment and the
population or individual at risk". I have underlined and highlighted three sets of
words, starting with "intensity". Part of exposure is the amount of some
potentially-hazardous agent. I’ve also
highlighted "time-averaged", as there is also a time component to exposure. Because
exposure takes into account both quantity and
duration, a higher exposure can occur if either a greater amount of an agent is present or
if a worker is in the presence of the agent for a
longer time. They are both important. I have also highlighted "appropriate interface"
because exposure must be measured at the
interface of the person with the surrounding environment. The best interface for an airborne
exposure might be if you could put some sort of
sampler directly in front of a worker’s nostrils. However, that's not very practical. Instead, we
often try to locate a sampler in what we call the
breathing zone of a worker by hanging the sampler on a worker’s collar so
that it is close to where the worker breathes,
without interfering with the breathing. A generic definition of dose is "the cumulative
amount of a property derived from an exposure that drives a biological response within the
exposed organism". I have highlighted the words "cumulative
amount" because a dose accrues over time as a
worker is repeatedly exposed. I’ve also highlighted the words "within the
exposed organism". This is important. While an exposure to an agent is outside the
person at the interface of the person with the
environment, dose is what gets inside. This is the main difference between exposure
and dose. This diagram is from a paper by Sexton and co-
authors. It shows the environmental health
paradigm. We think about agents being emitted into the
atmosphere of a workplace environment. There
is a source that emits the agent into the air or another medium, and there are pathways by
which the emitted agent moves through the
workplace before it reaches a person. The agent is present at some sort of
concentration, referred to as an exposure
concentration, in the vicinity of the worker. If the exposure is through the air, we can
measure the exposure concentration by sampling the air at an interface of the worker
with the environment. The agent can be inhaled or ingested or move
through the skin to form a potential dose inside
the body. Some of the agent that gets inside the body
may not be available for uptake, or some may
be immediately removed such as when you exhale a portion of a pollutant
that you have just inhaled. So, while this portion of the agent may be part of
a potential dose, it is not being applied to the
body. The portion that is applied to the body – for example, the fraction of incoming particles
that deposit in the lungs – is referred to as the
applied dose. Some of the applied dose may not be absorbed
into the body; it may be excreted instead. That portion which is absorbed is referred to as
the internal dose. From the internal dose, we move more into the
realm of toxicology where materials may be
delivered to certain organs or organ systems, they may or may not have a biological effect
after delivery, and those effects may or may not
be adverse. During the rest of the module, we will focus on
how to calculate exposure concentrations,
potential doses, and internal doses. From Sexton and coauthors, a more formal
definition of "exposure" is "contact of a biologic,
chemical, or physical agent with the outer part of the human body, such as
the skin, mouth, or nostrils". This is similar to
the definition that we looked at previously. "Exposure concentration" is "the concentration
of an environmental agent in the carrier medium
at the point of contact with the body". It’s an intensity or a quantity, as we’ve seen
before, at the interface of the environment with
the body. "Potential dose" is "the amount of the agent that
is actually ingested, inhaled, or applied to the
skin." And the "internal dose" is "the amount of the
agent absorbed, and therefore available to undergo metabolism,
transport, storage, or elimination". When we talk about exposure intensity, there
are various metrics that we can consider. The best metrics are ones relevant to the health
outcome associated with exposure to the agent
being investigated. We can consider metrics like mass
concentration, for example the milligrams of
some substance per cubic meter of air. Mass concentration is a common metric for
measuring exposure concentrations of gases,
vapors, and airborne particles. For number concentrations, the number of
microorganisms per unit volume or unit mass of
air, water, or food is a common example. Another example is the measurement of
airborne fibers, when air samples are analyzed
for the number of fibers per volume of air. Intensity is measured for sound pressure levels
using decibels. This is the basic unit for
measuring exposures to noise. Different units of concentration are used for
different media. In water, we use units such as
parts per million, parts per billion, or parts per trillion. These units refer to the mass of the
potentially-hazardous agent per mass of water.
Taking into account the density of water, it can be shown that one part per million of a
substance in water is equal to one milligram of
the substance per liter of water. Similarly, one part per billion is equal to a
microgram per liter, and one part per trillion is
equal to one nanogram per liter. For air, it’s different. For gases and vapors, we
use units of parts per million or parts per billion, but in air these units are considered to be on a
mole-per-mole basis. While these units sound
the same as the ones for water, they are not. An entirely different conversion is required to
convert from a measurement of mass per volume
of air to parts per million or parts per billion in air This conversion is based on the density of air
rather than the density of water. For airborne particles, mass per volume is
preferred with units such a milligrams per cubic
meter or micrograms per cubic meter. For dose, we can calculate both dose and dose
rate. Dose is typically specified in mass units
for chemical doses: the mass of a chemical taken in by a body
during a specified time interval. Dose rate is usually specified in units of mass
per time for chemical doses. Milligrams per day would be an example for
dose rate whereas dose would just be
milligrams. Dose and dose rate can be normalized to an
individual’s body weight, for example, milligrams
per kilogram of body weight for a mass-normalized dose or
milligrams per kilogram per day for a mass-
normalized dose rate. There are a variety of routes of exposure.
The three most common ones that we consider
are inhalation (breathing in), ingestion (swallowing water or food), and
dermal exposures. However, there can also
be exposures through the eye, referred to as ocular exposures, auditory
exposures through the ear, and whole body
exposures to hazards like vibration or radiation. Let’s discuss exposure and dose quantitatively. We can define cumulative exposure over a
certain time interval mathematically using this
formula. Cumulative exposure, E, is equal to the
concentration as a function of time multiplied by
the differential of time, dt, integrated over an interval of time from t1 to t2.
An integral is a calculation of an area under a
curve. If we think about a curve of concentration as a
function of time and then calculate the area
under that curve between two times of interest, that is the same as performing an integration
and would give you a cumulative exposure. Average exposure concentration, c-bar, can be
determined by taking the formula for cumulative
exposure, concentration times dt integrated from t1 to t2,
and then dividing it by dt integrated from t1 to t2. That is equal to the cumulative exposure, E,
divided by delta t, the duration of the exposure,
t2 minus t1. Rearranging terms shows that the cumulative
exposure is equal to the average concentration
times the duration of exposure. This is a good point, while we're thinking about
exposure being equal to concentration multiplied
by a duration of time, to think about acute versus chronic exposures.
Exposure is influenced by both quantity and
duration. Acute exposures to hazards typically have large
quantities and short durations, whereas chronic exposures typically have small
quantities and long durations. There are many types of exposures that are
acute and many types that are chronic. Sometimes, the same agent could be both an
acute and a chronic hazard. For instance, smoke from a structure fire can be
an acute exposure that can produce very
damaging health effects like smoke inhalation over a short period of time. However, smoke from
wood stoves, backyard fires, and other sources to which people are exposed may not create an
immediate health effect, but with chronic
exposure over a period of many years, those exposed may experience adverse health
outcomes including respiratory diseases and
even cancer. Because both acute and chronic exposures can
be important, when and how long we measure
exposure matters a lot. This is a plot of concentration on the vertical
axis versus time on the horizontal axis. If the
time interval we’re concerned about lasts from time t0 on the horizontal axis to time
t0 plus capital T, we might consider that to be
most of a workday. The plot of concentration versus time changes a
great deal as a function of time over this
workday. The area under the curve, the gray area, is the
cumulative exposure, the integral of
concentration multiplied by time. The horizontal line drawn across the gray area
that is labeled c-average is the average exposure concentration over the
time interval from t0 to t0 plus capital T. The area of the rectangle created by C-average
has the same area as the gray area under the
original curve. That's all an average is: it’s the value that
creates a box that possesses the same area as
the area under the original curve. The gray area is an example of the type of
measurement we would make to assess a
chronic exposure. For these exposures, we often take
measurements that last an entire workday. However, if we're more interested in an acute
exposure, the peak exposure that might cause
an acute health hazard, we might need to make a very short term
measurement that would pick up this peak
concentration and compare that short-term exposure
concentration to the risk from that exposure. So, the duration of exposure may be very
important depending on the type of health
outcome we're interested in. Moving on, potential dose is equal to the integral
from time t1 to t2 of concentration multiplied by a term we will refer to as the instantaneous
contact rate multiplied by the differential of time. The contact rate for inhalation exposures is the
inhalation rate or breathing rate that may be in
units of liters per minute, for example. For food or water, contact rate might be the
mass or volume of food or water consumed per
day. It is how much of the medium – air, water, or food – that the exposed person
takes in per unit of time. In occupational
hygiene, we frequently set potential dose equal to the average concentration times an
average breathing rate times the duration of time
exposed. Potential dose rate is dose divided by the
integral of time. This is equal to the average concentration times
the average breathing rate times the exposure
duration divided by the total time elapsed. The elapsed time may or may not be equal to
the time exposed. We will look at an example
of this in a little while. We will not spend much time with applied dose
and dose rate. We can calculate them from potential dose and
potential dose rate by applying a unitless
availability factor that accounts for some of the potentially-
hazardous agent brought into the body not being
made available. Examples include some portion of an agent that
may not be able to access the skin for a dermal
exposure or particles that are inhaled but then
immediately exhaled before they can come in
contact with the lining of the lungs. We frequently assume that the availability factor,
alpha, is equal to one, which means that the
applied dose and dose rate are equal to the potential dose and dose rate. This is certainly not always true, but we will
consider it to be true for the rest of this module. The internal dose rate, then, is the availability
factor times the integral from t1 to t2 of the
concentration, multiplied by the contact rate, times a term we’ll
refer to as the absorption factor, multiplied by
the differential of time. The absorption factor is the fraction of the agent
that is brought into the body that is actually
absorbed into the body. Some of the agent may be eliminated before it
can be absorbed, or it may pass through the
body without being absorbed. Frequently, we say that the internal dose is
equal to the potential dose times an average
absorption factor. Similarly, we set the internal dose rate equal to
the potential dose rate times the absorption
factor. This assumes, again, that the availability factor
is equal to one. Any of these doses or dose rates can be
normalized by body mass. We take the potential dose or dose rate, or the
internal dose and dose rate, and divide by the body mass to get mass-
normalized doses and dose rates. Let’s look at a couple of examples of these
calculations. In this first example, we will say that there is a
Pesticide X that is released into a room at a
kennel to kill fleas on a regular basis and is, therefore, in the air all the time. We want to
know the inhalation exposure to Pesticide X for
a worker in that room. We’ll assume that the worker weighs 50 kg and
that Pesticide X is absorbed through the lungs
at an average rate of 75%. We've made a measurement that indicates that
the air in the room contains a constant Pesticide X concentration of 0.1 milligram per
cubic meter. The worker will spend 8 hours per day in the
room and she breathes at a rate 18 cubic
meters of air per day while she is working. The two questions are, "What is the exposure
concentration?" and "What are the potential and internal and
mass-normalized potential and internal dose
rates to Pesticide X for this worker?" So, what is the exposure concentration? Well,
we’re given the exposure concentration in the
problem statement! The average concentration is equal to 0.1
milligram per cubic meter. We can multiply
that by 1,000 micrograms per milligram and say that the exposure concentration is also
100 micrograms per cubic meter. For the second part, what are the potential and
internal and mass-normalized potential and internal dose rates of the pesticide for the
worker? Potential dose rate is equal to the average
concentration times the breathing rate times the
duration of exposure divided by the elapsed time We know that the average concentration is 100
micrograms per cubic meter. We're given a
breathing rate of 18 cubic meters per day. The duration of exposure during each workday is
8 hours. The elapsed time, in this case, is one
day. We need to multiply one day in the denominator
by 24 hours per day to be able to cancel units. If we perform the calculation, we get a potential
dose rate of 600 micrograms per day, on each
workday. The internal dose rate is equal to the potential
dose rate times the absorption factor, which is
75% or 0.75. So, we take 600 micrograms per day, multiply it
by 0.75, and we get 450 micrograms per day. The mass-normalized potential dose rate is
equal to the potential dose rate divided by the
body mass. That is 600 micrograms per day divided by 50
kilograms, which equals 12 micrograms per
kilogram per day. Finally, the mass-normalized internal dose rate
is equal to the internal dose rate divided by
mass, which equals 450 micrograms per day divided by 50 kilograms, which equals 9
micrograms per kilogram per day. In a second example, we will consider a worker
in a facility that produces polymer fibers. This worker is exposed to an average of 7.9
milligrams per cubic meter of acrylonitrile vapor
on the job. He works 240 days per year, weighs 190
pounds, and breathes at a rate of about 22 liters
per minute during his 8-hour workday. The literature suggests that there is an
absorption factor of 52% for acrylonitrile when
it's inhaled. The first question posed is, "What is your
estimate of the worker's cumulative exposure to
acrylonitrile during his 8-hour workday?" The cumulative exposure can be calculated as
being equal to the exposure concentration times
the duration of exposure. That is 7.9 milligrams per cubic meter multiplied
by 8 hours, which yields a cumulative exposure
of 63 milligram-hours per cubic meter. The units for exposure may sound a little odd,
but they take into account both the
concentration and time elements of exposure. For part (b), the question is, "What is your
estimate of the annual internal dose rate in milligrams per kilogram per year for the
worker when exposed to acrylonitrile via
inhalation?" The internal dose rate is equal to the potential
dose rate times the absorption factor divided by
the body mass. That is equal to the average concentration times
the breathing rate multiplied by the time exposed and the absorption factor divided by the
time elapsed as well as the body mass. Substituting values for the variables gives 7.9
milligrams per cubic meter multiplied by 22 liters
per minute. The time exposed is 240 days during each one
year, which is the elapsed time, multiplied by 8
hours per day. The absorption factor's 0.52, and the body mass
of 190 pounds. Units conversions are needed, so we divide by 1,000 liters per cubic meter,
multiply by 60 minutes per hour, and also by 2.2
pounds per kilogram. When we carry out the calculation, we get a
mass-normalized internal dose rate of 120
milligrams per kilogram per year. To summarize, occupational hygiene is the
science, and to some extent the art, of anticipating, recognizing, evaluating, and
controlling workplace hazards. This is also referred to as the occupational
hygiene framework. Workers face a variety of
chemical, physical, biological, injury, and social/behavioral hazards at work, and
workers are the experts on their own work
environment. We need to talk to them if we are going to try to
understand the hazards that they face. Many hazards can be evaluated using
measurements or modeling, and by comparison
to occupational exposure limits. Options for managing those exposures or
controlling them are selected based on a hierarchy in which options that place the least
burden on individual workers are preferred. Exposure can be defined as the amount or
intensity of an agent at the interface between a person and his environment over a certain time
interval, and dose is the amount of the agent
brought into a person over a time interval. This lesson was created by the Midwest
Emerging Technologies Public Health and Safety Training Program, or METPHAST
Program, which is a collaboration among the University of Minnesota School of Public Health,
the University of Iowa College of Public Health,
and Dakota County Technical College. The METPHAST Program is funded by the
National Institute of Environmental Health
Sciences. This module's content is solely the responsibility
of its developers and does not necessarily represent the official views of the National
Institutes of Health. Thank you for viewing this module!

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