**Unit 4:**

**HAZARDOUS MATERIALS AND HUMAN HEALTH**

**OBJECTIVES**

**In this unit, you will learn:**

-   Why hazardous materials are a concern

-   What hazardous materials are

-   How hazardous materials affect the body

-   How hazardous materials enter and move through the environment

**HAZARDOUS MATERIALS IN THE UNITED STATES**

In the years since World War II, new technologies have developed at a
stunning pace. Nearly every business in our consumer society has grown
accustomed to daily use of manufactured products that offer us increased
convenience and efficiency. Hazardous materials or chemicals are used in
every industry today in some form or another.

This year alone, more than 1,000 new synthetic chemicals will be
manufactured for use in the chemical industry within the United States.
Some will require careful handling during manufacture, transport,
storage, use, and disposal in order to avoid causing harm to individual
users and handlers, other living things, and the environment.

many of these chemicals are not "biodegradable" (that is, they are able
to be broken down into their components by microorganisms); for such
chemicals in particular, the potential for adverse health effects can
continue for decades or even centuries.

We are becoming increasingly aware of the limited space that our planet
has to offer for the disposal of toxic products. Prior to release
reporting, the EPA noted that industry manufacturers alone in one year
discharged into the environment (air, land, and water) about seven
billion pounds of toxic substances. Numerous small businesses, such as
printing industries and vehicle maintenance shops, also released toxic
chemicals not included in these estimates. Few communities are eager to
have hazardous waste deposited in their "backyard." A lone barge loaded
with garbage made headlines that same year as it sailed the seas seeking
a place to leave its unwanted cargo. It seemed an apt expression of our
country's dilemma.

*While hazardous materials attract us by promising to make our lives
easier, they often confront us with complex problems---many of which
have no easy or immediate solutions.*

Naturally occurring toxic substances can also pose problems. For
example, ponds near a wildlife refuge in California became contaminated
by selenium, an element commonly found in alkaline desert soil. The high
level of selenium was the result of irrigation methods used at nearby
farms. Water removed the selenium from the soil, dissolved it, and
carried enough of the element to non-farm portions of the refuge to
threaten wildlife. As waterfowl ingested (ate) the selenium, deformities
were found more frequently in developing embryos. Naturally occurring
substances have sometimes led to expensive cleanup operations comparable
to those required for human-created hazardous waste.

Sometimes, the challenge posed by hazardous materials glares at us in
headlines and stories like these:

-   In Bhopal, India, 44 tons of methyl isocyanate gas spewed into the
    atmosphere, killing at least 1,700 people and injuring tens of
    thousands.

-   In a small Kentucky community, tank cars containing toxic substances
    derailed and burned. The fire caused a column of toxic smoke 3,000
    feet high that forced 7,500 area residents to evacuate.

-   In Florida, vandals broke the valves of chemical tanks at a local
    swimming pool supply company. The chemicals mixed to form a toxic
    acid, and a poisonous cloud of vapors sent 45 people to the
    hospital.

-   In Louisiana, up to 41,000 pounds of hydrobromic acid fouled part of
    the Mississippi River after two ships collided.

-   In Pennsylvania, a garbage truck operator found his load on fire and
    dumped it in a residential driveway; mixed chemicals, discarded by a
    high school science department, released cyanide vapors that sent
    100 people to the hospital.

-   Two New Jersey workers were killed and five injured by vapors
    inhaled as they cleaned a chemical mixing vat at a local company.

Often, however, problems posed by hazardous materials are less
clear-cut. Many of the effects attributed to toxic substances, such as
certain types of cancer, have multiple causes. In any single case of
illness or death, it is difficult to point the finger at a specific
instance of exposure to a particular hazardous material. In fact, one
study found traces of more than 200 industrial chemicals and pesticides
in members of an American sample group. Determining at what exposure
level each of these common substances becomes harmful to human health is
not only a scientific question but also a social, political, and
economic issue.

Our legal system seeks to control these materials at every level of
government---Federal, State, and local---but it is hampered by funding
limitations, debates over emerging technology, lack of definitive
research in certain areas, and competing rights and interests. Laws and
regulations at all three levels of government address various aspects of
the hazardous materials problem by specifying how chemicals must be
stored, what employees are told about chemicals they handle at work, how
chemicals are labeled, what containers are needed to transport specific
chemicals, and what emissions levels are acceptable from industries. In
each instance, the ***local*** government's role in regulating its own
hazardous materials problems is critical.

In 1986, the Emergency Planning and Community Right-to-Know Act made
history by requiring a local farm, industry, or small business that
stores a certain quantity of "extremely hazardous substances," as
defined by an EPA list, to report them to the SERC. This law, also known
as the Superfund Amendments and Reauthorization Act of 1986, or just
"Title III," includes provisions intended to help local-level planners
work with industry to identify and reduce risks from toxic chemicals,
and, if necessary, to seek corrective action through legal remedies. It
also creates new opportunities for citizens to identify and alter
potentially hazardous conditions in their communities. It is based on
the assumption that the more citizens know about local chemical hazards,
the better equipped they and their local governments will be to make
wise decisions about how risks associated with hazardous materials are
managed in their communities.

*Because Americans are exposed to toxic substances from so many sources,
it is often difficult to trace a health effect to a particular source.*

Chemicals being released into the environment (intentionally or
accidentally) by industries are major challenges facing our country
today. The chemical industry, plus manufacturers of hazardous chemicals,
transporters, and users of hazardous substances, when combined with the
many hazardous materials spills each year within the same community,
presents local government with a potentially serious threat to the local
environment and public health. Furthermore, automobiles emit nitrous
oxides (one source of "acid rain") and several air toxins. By becoming
informed about hazardous materials laws, issues, and protective actions,
local citizens can contribute to reducing their community's hazardous
materials threat.

**DEFINITION OF HAZARDOUS MATERIALS**

Hazardous materials, as discussed in this course, may be defined as:

A substance or material that the Secretary of Transportation has
determined is capable of posing an unreasonable risk to health, safety,
and property when transported in commerce. The term includes hazardous
substances, hazardous wastes, marine pollutants, elevated temperature
materials, materials designated as hazardous in the Hazardous Materials
Table (see 49 CFR 172.101), and materials that meet the defining
criteria for hazard classes and divisions in the standard.

Substances or materials, which because of their chemical, physical, or
biological nature, pose a potential risk to life, health, or property if
they are released.

A "release" may occur by spilling, leaking, emitting toxic vapors, or
any other process that enables the material to escape its container,
enter the environment, and create a potential hazard. Hazards are
classified in many different ways. The following introduces several
common terms:

1.  ***Explosive*** substances release pressure, gas, and heat suddenly
    when they are subjected to shock, heat, or high pressure. Fourth of
    July celebrations use many types of explosive substances that
    require careful storage and handling to avoid injury.

2.  ***Flammable*** and ***combustible*** substances are easy to ignite.
    Paint thinners, charcoal lighter fluid, and silver polish are all
    highly flammable. Oxidizers, which will lend oxygen readily to
    support a fire, and reactive materials, which are unstable and may
    react violently if mishandled, pose related hazards.

3.  ***Poisons*** (or toxic materials) can cause injury or death when
    they enter the bodies of living things. Such substances can be
    classified by chemical nature (for example, heavy metals and
    cyanides) or by toxic action (such as irritants, which inflame
    living tissue, and corrosives, which destroy or irreversibly change
    it). One special group of poisons includes ***etiological***
    (biological) ***agents***. These are live microorganisms, or toxins
    produced by these microorganisms, that are capable of producing a
    disease.

4.  ***Radioactive materials*** are a category of hazardous materials
    that release harmful radiation. They are not addressed specifically
    in this course.

These categories are not mutually exclusive. For example, acids and
bases are listed as corrosive materials, but can also act as poisons.

**HOW HAZARDOUS MATERIALS HARM THE BODY**

*Absorption and injection are two routes of entry that occur through
direct skin contact with a hazardous material.*

Chemicals and hazardous substances may enter the body by several routes,
and the nature and onset of signs and symptoms may vary accordingly.
Gases, vapors, and aerosols, when inhaled, may be absorbed through any
part of the respiratory tract, from the mucosa of the nose and mouth to
the alveoli of the lungs. The eye may also directly absorb them. Aerosol
particles larger than 5 **micrometer (μm**) tend to be retained in the
upper respiratory tract, while those smaller than 1 µm tend to be
breathed in and out again, although some of these smaller particles may
be retained. Droplets of liquid and, less commonly, solid particles may
be absorbed through the surface of the skin and mucous membranes. Toxic
compounds with a characteristic action on the skin can produce their
effects when deposited on the skin as solid or liquid particles.

Chemicals or hazardous substances which penetrate the skin may form
temporary reservoirs so that delayed absorption may occur. Even the
vapor of some volatile chemicals and agents can penetrate the intact
skin and intoxication may follow. Wounds or abrasions (even minor
injuries caused by shaving or by chemical depilation) present areas
which are more permeable than intact skin. Chemicals and hazardous
substances may contaminate food and drink and so be absorbed by the
gastrointestinal tract. The penetration of chemicals and hazardous
substances by these various routes may not be accompanied by irritation
or damage to the surfaces concerned.

Methods of exposure to these chemicals are called ***routes of entry***.
They are:

1.  Absorption

2.  Ingestion

3.  Injection

4.  Inhalation

```{=html}
<!-- -->
```
1.  **Absorption (through the skin or eye)**

> If a child were to walk barefoot through contaminated soil, the
> contaminant would contact the skin of the foot. This could cause mild
> skin irritation, or more serious problems like burns, sores, or ulcers
> on the outer layers of the skin. Contact with a substance may also
> occur by spilling it on the skin or brushing against a contaminated
> object.
>
> Depending on the substance and the condition of the skin, the
> contaminant might also be absorbed through the skin and poison the
> body. While some chemicals are not absorbed easily unless the skin is
> cut, others are absorbed quite readily regardless of the skin's
> condition. When you are using a material that bears instructions
> recommending the use of gloves, this is to prevent skin contact or
> absorption through the skin (also called ***dermal*** exposure).
>
> When you work with chemicals, it is particularly important never to
> put your hand to your eye. Eyes are particularly sensitive to toxic
> substances; since capillaries are near the surface, the substance can
> enter the bloodstream more readily. Eye contact with toxic substances
> can cause irritation, pain, or even blindness.

2.  **Injection**

> The most familiar example of injection is that of shots given to
> administer medicine, in which the skin is punctured with needles so
> that a substance can enter the body. Injection can also occur
> accidentally. For example, if a contaminated can or a piece of glass
> that had been in contact with a contaminant cut the skin, the
> contaminated substance could be injected into the body. This is a very
> powerful means of exposure because the contaminant enters the
> bloodstream ***immediately***.

*When we ingest (eat) or inhale a substance, the body tries to filter it
out through internal defenses. If there is enough of the substance,
these filters are overwhelmed.*

3.  **Ingestion**

> If we eat a substance that contains a harmful material, that substance
> enters our bodies by means of our digestive system. An example of
> inadvertent ingestion is a battery factory employee who eats lunch in
> the work area and ingests inorganic lead that has contaminated a
> sandwich. A more common instance is the child who puts a toxic
> substance in his or her mouth out of curiosity. We may also ingest
> residue from chemicals that have been added to our food to kill germs
> or parasites.

4.  **Inhalation**

> It is also possible to be contaminated by toxic substances when we
> breathe them into our lungs. The amount of air inhaled in a workday
> can be extremely large, so if we work or live in a contaminated area,
> we can be exposed to significant quantities of a substance in this
> way.
>
> Some chemicals have excellent warning properties that let us know when
> they are in the atmosphere. There is the well-known "rotten egg" smell
> of hydrogen sulfide, for example. But at high concentrations of this
> gas, our sense of smell is quickly lost. Many toxic substances, such
> as carbon monoxide, are both colorless and odorless, providing us with
> no sensory clues that we are being exposed to anything unusual.

**PATHWAYS OF EXPOSURE**

If we consider these routes of entry, it is possible to think of a
number of ways in which contaminants escaping into the environment from
their ***source*** may reach a living plant or animal, or
***receptor***. Each specific route a chemical might travel from a
source to a receptor is called an ***exposure pathway***.

The pathway may be either ***direct*** or ***indirect.*** If an open
toxic waste dump were near you, you could inhale the vapors from the
toxic material, or your skin could contact toxic contaminants if you
walked through the substance. These are ***direct*** means of exposure.
The substance can also reach you by ***indirect*** pathways. For
example, toxic vapors or particles from a site at which hazardous waste
has been illegally discarded could be carried in the air to a cornfield
and deposited on the crop as it rains. You ingest some of the toxic
chemical as you eat the corn; or perhaps a farm animal eats the corn and
you later eat meat from that animal.

Another pathway might be through drinking water. When rain falls and
passes through polluted soil, it carries chemicals deeper into the earth
as well as horizontally across the surface of the soil. If they are able
to move far enough---which depends on the geology of that particular
area---they could contaminate the groundwater. The contaminants could
also be carried along the land by means of ***surface runoff***, water
that moves along the top of the soil, until they reach a recreational
pond where children swim. Now there would be another opportunity for
dermal contact.

**ASSESSING RISK**

How much ***risk*** is associated with a particular source depends on
the characteristics of the source, the availability of pathways for it
to reach the receptor, and the characteristics of the receptors. No
single piece of information alone is sufficient, and incomplete
information can be highly misleading. Among the key questions that must
be asked in determining risk are the following:

1.  What are the ***hazardous properties*** of the substance? What
    effects can it have on living things or on the environment? (To
    answer this question, it is often necessary to consider the state of
    research on the substance, and how much is really known about it.)

2.  How ***much*** of the substance exists at the source, and in what
    ***concentration***? A higher quantity or concentration of a toxic
    substance is more dangerous. However, the risk posed by a highly
    concentrated toxic substance entering the environment depends on the
    pathways available to it, and to what extent the concentration is
    reduced by the time it reaches receptors.

3.  In what ***form*** is the substance? Whether the substance is in
    large blocks or tiny particles, or whether it is a liquid or a
    vapor, will be important in determining not only how it might
    travel, but also how it could contact and enter the body.

4.  What are the ***chemical and physical characteristics*** of the
    substance? These characteristics determine in what environmental
    pathways it is likely to move and how rapidly. They include, for
    example, whether the substance can easily dissolve in water.

5.  How is the substance ***contained***? If the chemical is in old,
    rusting containers that can leak, the danger is clearly greater than
    if the container is solid and appropriate to the substance.

6.  What ***pathways of exposure*** exist? When scientists study the
    risk in any particular situation, they look at all the ways a
    contaminant could reach the population at risk and make measurements
    to see how much of it is moving through each identified path. For
    example, if the source were near a stream, water samples would be
    taken at several places to see what level of contamination exists at
    different distances from the source.

*A possible exposure pathway involving the food chain: toxic fumes and
particles from a waste dump are carried through the air to a cornfield;
corn grown in this field is fed to an animal, which is later processed
for human consumption.*

7.  Where is the population ***located*** in relation to the source?
    Distance is a critical factor. For example, if you are far
    downstream from a place where toxic waste is entering a waterway,
    you may have little risk because the substance is so diluted. Closer
    to the source there might be a high enough concentration to pose a
    real problem.

8.  What are the ***characteristics*** of people who are at risk? The
    susceptibility of any individual to a toxic substance varies
    depending on age, weight, sex, and individual sensitivity.

9.  How long does the exposure to the chemical last? Its ***duration***
    is another key factor in determining risk. Are receptors exposed for
    only a few hours at a high level (such as when a contaminated air
    plume passes over a home), or at a low level over a number of years
    (such as when groundwater supplying a well becomes contaminated)?

The analysis of a situation to determine the level of risk inherent in
that situation is called ***risk assessment.*** A risk assessment is
conducted by scientists from many different disciplines and uses data
about a chemical's effects combined with research into the particular
situation to get a clear picture of the risk posed. A decision will then
be made as to what action, if any, is needed to remedy the situation.
This is called ***risk management***.

**TOXIC MATERIALS IN THE BODY**

A poison, or toxic substance, may be defined as ***a chemical that, in
relatively small amounts, produces injury when it comes in contact with
susceptible tissue***. Clearly, the phrase "relatively small amounts" is
less than precise, but this uncertainty is necessary because of the wide
variance in the amount of each chemical needed to have an effect. A
substance is generally not thought of as toxic if it is unreasonable to
expect that a person would be exposed to the amount necessary to cause
injury. A "susceptible" tissue is defined as that part of the body which
is injured after exposure to that particular substance.

**Toxic Effects**

There have been many attempts to categorize toxic effects and to define
them in various ways. Generally, the terms "acute" and "chronic" are
used to delineate between effects on the basis of severity or duration.

The first method or ***acute*** ***exposure*** is the exposure to a
hazardous substance over a short period of time or at a high dose. A
reaction to a chemical can occur at the time of exposure, and might
include vomiting, eye irritation, or other symptoms that often may be
readily linked to a chemical exposure. These are ***immediate and
adverse*** effects.

The second method or ***chronic exposure*** is the exposure to a
hazardous substance over a long period of time. If a carpenter used a
stripper regularly and breathed in a little of it 8 hours a day for 40
years, a chronic exposure would result. This type of exposure occurs
when a person is repeatedly exposed to the same chemical or hazardous
substance over a long period of time at very low levels.

Similarly, the term chronic effect is often used to cover only
carcinogenicity, teratogenicity, and mutagenicity (terms to be discussed
below). These effects are obviously a concern in the workplace, but
again do not adequately cover the area of chronic effects, excluding,
for example, blood dyscrasias (such as anemia), chronic bronchitis, and
liver atrophy (wasting, losing function, or size).

**Local and Systemic Effects**

Local effects from chemical exposure occur at the site of contact, i.e.,
eye irritation, skin burns or blistering, respiratory distress, or
pulmonary edema. Systemic effects occur at a location distant from the
point of contact, i.e., liver, CNS, heart, or kidneys. These effects may
occur years after a single high level of exposure, or as the result of
chronic exposure. Systemic effects are often more difficult to trace to
their cause, and can include organ damage, respiratory diseases, and
other illnesses in an exposed population.

Certain toxic substances produce their long-term effects by altering the
genetic code, or DNA, which tells the body's cells to perform certain
activities. Three categories of effects can result from such substances:

1.  A ***carcinogenic*** effect is an increase in an individual's risk
    of contracting cancer.

> \(a\) It has been evaluated by the International Agency for Research
> on Cancer (IARC), and found to be a carcinogen or potential
> carcinogen; or
>
> \(b\) It is listed as a carcinogen or potential carcinogen in the
> Annual Report on Carcinogens published by the National Toxicology
> Program (NTP) (latest edition); or

\(c\) It is regulated by OSHA as a carcinogen.

2.  A ***mutagenic*** effect is a permanent change in the genetic
    material (DNA), which may be passed along to later generations.

3.  A ***teratogenic*** effect is an increased risk that a developing
    embryo will have physical defects.

Determining what level of exposure causes these effects requires
laboratory research under controlled conditions. Even then, results must
be ***extrapolated*** from laboratory animals to humans. That is,
scientists must make assumptions and apply formulas to decide what their
experiments tell them about ***human*** exposures.

Another way to classify a hazardous substance or chemical is by their
physiological effects. This classification includes the following major
groups:

1.  ***Irritants*** are chemicals which are not corrosive, but which
    cause a reversible inflammatory effect on living tissue by chemical
    action at the site of contact. A chemical is a skin irritant if,
    when tested on the intact skin of albino rabbits by the methods of
    16 CFR 1500.41 for 4 hours exposure or by other appropriate
    techniques, it results in an empirical score of five or more. A
    chemical is an eye irritant if so determined under the procedure
    listed in 16 CFR 1500.42 or other appropriate techniques.

2.  ***Corrosives*** are chemicals that cause visible destruction of, or
    irreversible alterations in, living tissue by chemical action at the
    site of contact. For example, a chemical is considered to be
    corrosive if, when tested on the intact skin of albino rabbits by
    the method described by the U.S. Department of Transportation in
    appendix A to 49 CFR part 173, it destroys or changes irreversibly
    the structure of the tissue at the site of contact following an
    exposure period of 4 hours. This term shall not refer to action on
    inanimate surfaces.

3.  ***Asphyxiants*** can be physical or chemical. ***Physical
    asphyxiants*** are gases or vapors that dilute or displace oxygen
    normally in the atmosphere. (Vapors from flammable and combustible
    liquids displace oxygen in the environment, being heavier than air.)
    ***Chemical asphyxiants*** are chemicals that prevent the cells from
    taking up or transferring oxygen in the body or to the tissues.
    Carbon monoxide is a well-known asphyxiant, which chemically "ties
    up" the hemoglobin in the blood so that the body's metabolism slows
    and stops.

4.  ***Central Nervous System (CNS) Depressants*** affect the nervous
    system. This broad category includes vapors from most anesthetic
    gases, depressants, and organic solvents (a general category that
    includes most household cleaners as well as many paints, glues, and
    adhesives). Some CNS depressants produce a feeling of dizziness or
    giddiness. More severe effects (including death) can also result.

5.  ***Systemic Toxicants*** dramatically affect specific organ systems.
    For example, mercury vapor, which Victorian hat makers had to inhale
    regularly when mercury was used in making hats, causes a serious
    nervous system disorder which could lead to insanity. (The "Mad
    Hatter" in Alice in Wonderland suffered from an occupational
    illness.)

6\. ***Sensitizers*** are chemicals that cause a substantial proportion
of exposed people or animals to develop an allergic reaction in normal
tissue after repeated exposure to the chemical. (Examples are
formaldehyde, poison ivy, and poison oak. Some epoxy resins and
polyester resins can cause many people to have a ***sensitivity
reaction*** and become ill.)

Many chemicals can have multiple effects. For example, xylene, commonly
used in paint, is both an irritant and a CNS depressant.

Symptoms of toxic exposure include a broad range of reactions: chronic
coughs, difficulty in breathing, skin ulcers, diarrhea, irregular
heartbeat, headaches, dizziness, chest pain, sore eyes and skin,
difficulty in sleeping, lack of appetite, weight loss, nausea, tremors,
and many others.

However, the same symptoms can result from many other causes as well.
Tracing a particular reaction to a specific source can be a challenge to
even the most experienced environmental toxicologists, allergists, and
industrial hygiene specialists. This is further complicated by the fact
that many effects are delayed, and are apparent only later in life. The
individual experiencing the symptom may no longer live near the original
source, or may not even know that the exposure occurred.

*At low enough exposure levels, a toxic substance will produce no
observable harmful effects. As the dose increases, so does the potential
for harm. For every substance---even table salt---there is a lethal
dose.*

**Internal Defenses**

When the body is exposed to a hazardous chemical, its internal defenses
try to remove the unwanted substances. The primary internal defense is
***excretion*** of the contaminant with other wastes in the feces or
urine. Prior to excretion, primarily the liver and kidneys filter
wastes. As a result, these two organs are both subject to damage from
toxic substances, storing in their tissues what they are unable to break
down. Portions of the lungs contain cilia, which try to remove particles
so that they can be coughed out. Particles that are too small or cannot
be removed for other reasons will remain as deposits in the lower part
of the lungs (alveoli), where they can cause scaring, fibrosis, or
cancer.

Other body defenses against toxic substances are ***breathing*** and
***sweating***. When an intoxicated person has the smell of alcohol on
his or her breath, the smell indicates that the body is exhaling
material it has no use for. Tears also remove contaminants that enter
the eyes. However, these defenses contribute only a small amount to the
body's detoxification (that is, its attempt to rid itself of toxic
substances).

The body's ability to defend itself against toxic substances varies with
the individual. Small children are liable to be more affected by the
same amount of a substance than are larger or older persons. Elderly
individuals also may have less ability to remove toxicants from the
body. Gender can be a factor in toxic responses; for example, some
cancers are sex-linked (such as prostate and ovarian cancers). Personal
hygiene and the overall health of an individual can also adversely
affect the body's ability to process certain toxic substances. For
example, a smoker is likely to be much more susceptible to lung cancer
if he or she has also been exposed to high levels of radon gas.

Exposure to a poison becomes a problem when the material is of a type
that inner defenses cannot break down and remove, or when there is more
of it than the body can handle. In these instances, ***antidotes*** are
available for a limited number of substances. However, only about 20
antidotes are in existence for the thousands of poisons in the
world---and each antidote may work for only a few poisons.

Clearly, the safest barrier to toxic exposure is the ***prevention*** of
exposure. This is why it is so important for citizens to be aware of the
threat posed by hazardous materials in their own home and community, and
to learn to minimize or eliminate unnecessary exposure.

**STUDIES OF TOXIC EFFECTS**

Scientists determine what levels of exposure in human beings will
produce observable symptoms by two types of studies.
***Epidemiological*** studies use data on how toxic substances affect
human populations. This type of study might compare the number of
workers exposed to a certain substance who develop lung cancer to those
who develop it in the rest of the population. Other ***clinical***
studies test the effects of concentrated doses of substances on animals
or animal tissue.

*Scientists conduct two kinds of studies---epidemiological and
clinical---to determine what levels of exposure to a hazardous substance
will produce observable symptoms.*

A basic principle of research on toxic substances is that the
seriousness of the effect a poison has on the body increases as the
***dose*** increases. Theoretically, there is a ***threshold*** for
exposure to each poison. Beneath the threshold, the dose is so small
that no harmful effect will occur. As the dose increases, there is a
point at which there is an effect, but the animal can compensate for it
by internal healing, and no permanent injury will occur.

Beyond that, there is a dose at which the animal cannot repair itself
from the damage and disease results. Finally, at the upper limit of the
curve, death occurs.

#### Terminology

The terminology used in this unit to explain the dose response
relationship is as follows:

*Dose.* The dose is the quantity of the compound received by the
subject.

*LD~50~.* The LD (lethal dose)~50~ is the dose which kills 50% of the
exposed population.

*ID~50~.* The ID (incapacitating dose)~50~ is the dose which
incapacitates 50% of the exposed population.

*Ct (Concentration time).* The Ct is a measure of exposure to a vapor or
aerosol. The concentration in the air and the time of exposure govern
the dose received, as dose rate of respiration. It is assumed that, when
the product of concentration and time is constant, so is the biological
effect over a limited range of concentration and time. For very short or
long exposures the biological effect may vary. Concentration is
expressed as milligram per cubic meter (mg/m^3)^ and time as minutes, so
that the concentration time (Ct) is expressed as milligram-minutes per
cubic meter (mg-min/m^3)^.

*LCt~50~.* The LCt (lethal concentration time)~50~ is the Ct which will
kill 50% of the exposed population.

*ICt~50~.* The ICt (incapacitating concentration time)~50~ is the Ct
which will incapacitate 50% of the exposed population.

***Death would occur if sufficient quantities of any substance were
taken into the body.*** For example, if a large group of people with
similar characteristics ate half a pound of table salt, half of them
would probably die. Through experiments, scientists try to establish the
particular dosage of chemical (in mass per kilogram of body weight) that
will result in the death of half the test animals: that is the ***Lethal
Dose*** ***for 50% or LD~50~.*** They also try to establish the point at
the other end of the curve at which there is no observable effect from
the substance on the animal. This is called the ***NOAEL: No Observable
Adverse Effect Level.***

Once the LD~50~ for a substance has been established by repeated
experiments with animals, it must be extrapolated to determine what the
LD~50~ would be for humans. This means adjusting the results to apply to
human body weight and similar characteristics. But a toxic substance
often has different effects on different species, so tests on animals
cannot predict the exact effect that the substance will have on a human
population. As a result, scientists are usually quite conservative in
their estimates, which mean that they assume that the smallest dose that
causes an effect in animals will also cause an effect in humans. In
addition, scientists study the effect of a substance on human
populations wherever statistics are available.

*A toxic substance will sometimes combine with another substance to
create a new chemical. The potential for harm of this new chemical can
be greater or lesser than that of its individual components.*

Another uncertainty associated with the LD~50~ concept is that most
LD~50~ data is gained from acute exposure (single dose) testing rather
than by chronic exposure. Extrapolation from these studies is
complicated by the fact that chemicals are sometimes distributed
differently in the body when the exposure is chronic; for example, a
different target organ may be attacked, or the material may be excreted
more easily.

Given these uncertainties, it is understandable why there is often
considerable debate about what constitutes a "safe" level of exposure.
For most substances, agency experts extrapolate conservatively from the
NOAEL to set exposure limits for humans. OSHA uses ***Permissible
Exposure Limits (PELs),*** while the American Conference of Governmental
Industrial Hygienist (ACGIH) uses ***Threshold Limit Values (TLVs)***,
to define the workroom air concentration that is considered a safe upper
limit of exposure. For carcinogens and mutagens, however, there is
considered to be no such "safe" exposure limit for regulatory purposes.
Every exposure carries some risk.

**HAZARDOUS MATERIALS IN THE ENVIRONMENT**

*The movement of contaminants within a medium such as air, groundwater,
or soil is known as transport.*

Hazardous materials can enter the environment either from a specific
source that can be pinpointed, known as a ***point source***, or from
sources that are more spread out, known as ***area sources***. A factory
smokestack and the flow of toxic waste from a pipe to a stream are point
sources, while the liquid runoff from a field in which pesticides were
used is considered an area source.

Contaminants behave differently in the environment depending on their
***physical state***. A solid may stick to surfaces, scatter, or form a
dust cloud; a liquid may seep into the ground, flow along the ground, or
vaporize and become a gas; a gas will expand and be carried by the wind.
Some chemicals are ***volatile***, meaning that they evaporate easily.
Such a chemical may enter a stream as a liquid but rapidly become an
***air*** pollution problem.

A non-volatile chemical entering the same stream at the same point may
behave quite differently. A ***soluble*** chemical is one that will
dissolve readily in water, and would be carried by the stream. Soluble
chemicals tend to be ***mobile***, meaning that they will move rapidly
in the ground because they can be easily dissolved in groundwater.
Another chemical might be more likely to ***adsorb*** to soil particles,
becoming attached to particle surfaces. Such a chemical would attach to
particles in the stream and eventually settle at the bottom. If the
chemical were a ***persistent*** one, which resists breakdown in the
environment, it might remain there for some time in the same form, while
bacteria might break down a less persistent chemical. This breakdown is
called ***biodegradation***, and is an important risk management
concept. Sometimes it is possible to ***increase*** biodegradation so
that materials lose their harmful properties more readily.

*The process of chemical breakdown, or biodegradation, can cause
materials to lose their harmful properties and, in effect, "disappear."*

*Certain chemicals tend to become more highly concentrated as they move
through the food chain. This process is known as biomagnification.*

Contaminants enter any of the various ***media***---air, groundwater,
surface water, or soil---and move as a mass along with the general flow
of that medium. This movement of contaminants within a medium is called
***transport***. Substances in transport also tend to spread out as they
move, becoming diluted to a varying extent by the medium. This generally
reduces the concentration, and therefore lowers the level of hazard.

Once a toxic substance is released into a medium, a number of different
processes can occur:

1.  The substance moves in a pathway determined by its own
    characteristics and those of the medium that is carrying it.

2.  The substance spreads out or disperses, reducing the level of
    hazard. This means of reducing risk is not always reliable or
    consistent, however. For example, there may be periods of low flow
    in streams when the volume of water is reduced and less dilution
    occurs.

3.  The material may change chemically or break down into other elements
    or compounds. Sometimes a contaminant will combine with another
    substance to become a more dangerous chemical; at other times it
    will be rendered less harmful by the encounter. Some chemicals have
    a ***synergistic*** effect. (A synergistic effect is the effect of
    two chemicals acting together causing a greater effect than the
    simple sum of their effects when acting alone.)

4.  A chemical may also ***potentiate***. (Potentiation is the ability
    of one or more chemicals to increase the toxicity of another
    chemical to cause greater harm than the total effects of the two
    expected reactions.) An example is an alcohol. When mixed it will
    potentiate the effects of many chlorinated hydrocarbons.

5.  The reverse is also possible regarding a chemical. It may present an
    antagonistic effect. Or the results of two or more chemicals may
    lessen the total effects of their combined exposure in the body. An
    example might be one taking aspirin to lessen the effects of an
    alcohol (wine or beer) exposure from the night before.

6.  A toxic substance may move from one medium to another (for example,
    evaporating from water into air).

7.  Toxic substances can build up in the food chain. Organisms can
    absorb contaminants such as pesticides in a process known as
    bioaccumulation. These contaminants are later released into another
    organism that eats that animal or plant. Certain chemicals also tend
    to become more concentrated as they move up the food chain. (For
    example, toxic concentrations may be higher in a bird that ate
    insects containing poison than in the insects themselves.) This is
    known as ***biomagnification***. Often, an important part of
    understanding a chemical's risk to humans is being aware of how a
    particular contaminant will move through a food chain and how each
    animal or plant in the chain may be affected.

The way a pollutant is ***transformed*** by chemical reactions and
***transported*** through the environment is called its ***fate***. As
we have seen, the fate of chemicals released at the same site may be
extremely different.

**The Movement of Contaminants in Different Mediums**

Hazardous substances move and disperse differently, depending on the
medium in which they are deposited. Regulators set standards for
exposure in each media separately, trying to take the unique features of
each one into account. There are four ***transport mediums*** in which
contaminants travel.

***Air***

Hazardous chemicals can enter the atmosphere from a point source (such
as an industrial stack), or from an area source (such as the evaporation
of volatile compounds from hazardous waste sites). A major factor
affecting the level of contaminants in the air is the rate of
dispersion, which is affected by both weather and ***topography*** (the
shape of the land, including buildings). With a good, strong wind, air
pollutants are dispersed more rapidly; when the air is calm, contaminant
concentration increases. As a rough rule of thumb, contaminant levels
are ***halved*** when wind speed is doubled. (This rule of thumb assumes
no effect from topographical features.)

*In urban locations, a turbulent "mixing area" of air exists which helps
diffuse contaminants. This is due in part to the irregular surfaces of
various-sized buildings and hot and cold spots created by contrasting
materials such as concrete and grass.*

Contaminant levels are also affected by the amount of ***vertical
mixing*** that occurs. Normally, temperature decreases with height; we
have all noticed how much colder the air is on top of a high mountain.
In urban areas, under such temperature conditions, a turbulent ***mixing
area*** of air exists, characterized by swirls, gusts, updrafts, and
downdrafts. This movement is partly attributable to the irregular
surfaces of small and tall buildings, and hot and cold spots created by
contrasting materials (asphalt or concrete vs. grassy park areas).
Polluted air is carried upward and dispersed, while relatively cleaner
air moves downward. The net result is that pollutants move up and away
from us.

Under weather conditions in which temperature increases with height,
much less vertical mixing occurs, and pollutants can grow thick in the
breathing zone. Such conditions typically occur when a warm air mass
moves over a cooler layer of air. In areas with basin-like topography,
such as Los Angeles, high-pressure systems can develop in which air
above the 2,000- to 3,000-foot level dips and warms, while the air near
the earth stays relatively cooler. Hazardous situations called
"episodes" can last for days in such areas, often confining persons with
respiratory difficulties such as emphysema indoors.

*Under certain weather conditions, a "temperature inversion" can occur
which traps contaminants and can promote unusually high levels of
exposure.*

The height of the source can also affect the distribution of pollutants
in air. For ground-level releases, the highest concentrations are almost
always near the source; for elevated sources such as stacks more than 30
feet above ground, however, the highest concentrations may be further
downwind from the source. The size of the particles emitted is also
relevant; larger particles are more likely to settle out near the
source, while small ones will travel further in the air.

***Groundwater***

Groundwater, defined as water moving through soil and rock, is a common
route for chemical movement. The source of groundwater contamination can
include surface ***impoundments*** in which hazardous materials are
disposed or stored, such as ponds and lagoons, leaking underground
storage tanks, or any spill where contaminants can seep downward. The
type of soil configuration is crucial in groundwater contamination. Some
soil layers, such as clay, are harder for contaminants to move through
(less permeable) and can protect the underlying groundwater.

While contaminants in rivers or streams are generally churned and
diluted by movement as they are in the air, contaminants can move great
distances in groundwater without dilution. Also, chemicals in the
groundwater last longer; chemicals cannot evaporate, and they resist
breakdown in the absence of air and light. It is difficult and sometimes
impossible to purify contaminated groundwater.

***Surface Water***

Surface water includes oceans, rivers, lakes, streams, or any
aboveground water source. It may be contaminated by industrial and
sewage discharge pipes, chemical spills, or hazardous waste landfills.

The concentration of chemicals in surface water depends on the amount of
the substance entering the water, its properties, and the water's rate
of flow. Chemicals that are heavier than water, which include PCBs and
dioxin, settle out in the sediment at the bottom and can remain there
for long periods, while lighter chemicals will flow with the stream.
Exposure to light, oxygen, and organisms will break down some chemicals;
others, such as heavy metals (lead and mercury), are ***persistent***
and do not break down easily. Substances that are persistent may be
transported to estuaries (areas where rivers meet oceans) and accumulate
in shellfish and fish.

***Soil***

Soil may become contaminated through dumping, spills, and other sources.
Rainwater leaches some contaminants from the soil and carries them to
the groundwater; other contaminants remain near the surface, where they
can affect human health by entering the food chain (ingestion), emitting
toxic vapors (inhalation), or rubbing onto the skin of children playing
in the dirt (dermal absorption). Because contaminated soil is a basic
contaminant medium that affects other media, it is of considerable
importance.

*Surface water generally dilutes contaminants as it moves; they tend to
break down as they contact air and light. Contaminants resist breakdown
in groundwater and do not disperse readily because there is nowhere to
go.*

Once a contaminant has gained a foothold in one medium, it may be
released into others as well. Whether this happens or not depends on the
contaminant's characteristics and pathways available in the environment.
Contaminants evaporate from the soil or water and enter the air through
***volatilization***. Contaminants can also ***leach*** from the soil
and enter water, or be blown by the ***wind*** and become airborne
particles. When it rains, contaminated ***runoff*** from the soil can
enter a stream. Therefore, it is impossible to simply place a chemical
in one medium and forget about it. Careful thought must be given to how
it may be released into other media.

**SUMMARY**

While we often associate our hazardous materials problems with industry,
naturally occurring toxic substances, products used in average
households, and automobiles all contribute to our country's challenge.
Materials may be considered hazardous for different reasons---for
example, some are liable to explode or burst into flame easily, while
others can poison us. Poisons can enter our bodies by absorption,
ingestion, inhalation, or injection, through various environmental
pathways in different mediums.

Predicting how much of a substance will actually reach us from a
particular source is complicated. Chemicals often follow complex
pathways through the environment, leaving one medium to enter another,
and being transformed and transported in different ways by each medium.
They may either disperse harmlessly or become concentrated as they move
through the food chain.

*Numerous sources contribute to groundwater contamination. Because
groundwater is a source of drinking water for half the Nation's
population, and because it is so hard to clean up, protecting this
resource from contamination is a major concern.*

While our bodies have many internal defenses against poisons, these
defenses can be overwhelmed. Whether we are exposed to chemicals all at
once or gradually over time, we can reach a threshold at which harmful
effects are noticeable---sometimes years after our first exposure. To
ensure our health and safety, we should take reasonable precautions to
limit our potential exposure to hazardous substances.

**HAZMAT TEASER**

(Answers are on pages C-2 and C-3)

The driver of a gasoline tanker unloading at a local service station
leaves his truck unattended for a short period of time. When the driver
returns to his truck, he finds that the service station's underground
storage tank has overflowed and more than 1,000 gallons of gasoline are
covering the ground and flowing down the street.

A strong odor of gasoline covers the entire spill area and extends
downwind for several blocks. People who live downwind of the incident
notice the gasoline odor and become concerned: a well driller in the
area reports the odor to the local 911 dispatch center, a nursing home
in the area starts to move patients inside from an outdoor recreation
yard, and teachers from a nearby elementary school begin to bring
students indoors from recess. The driver of the tanker works furiously
to shut off the truck's delivery valves to halt the overflow; in the
process, his pants and shoes become drenched with gasoline. Down the
street from the service station, the gasoline is running into a roadside
drainage ditch and soaking into the ground. Some of the product has also
reached a small stream that flows through the area.

1.  How would you assess the risk from this incident? What are some of
    the things to take into account when making your risk assessment?

2.  What routes of exposure might affect the people downwind of the
    spill? What routes of exposure might affect the driver of the
    tanker?

3.  Does the potential of a long-term exposure threat exist? If so, what
    type of exposure would this be?

4.  What transport media (air, surface water, groundwater, soil) are
    involved in this incident? How are they involved?

5.  Do the people in the nursing home and the students in the elementary
    school face any ***increased*** threats from exposure to gasoline
    fumes? If so, why?

**CHECK YOUR MEMORY**

(Answers are on page C-4)

1.  While spraying pesticides, a farmer wears long pants, a long-sleeved
    shirt, and gloves. The farmer is trying to prevent contaminants from
    entering the body by:

    a.  Absorption

    b.  Injection

    c.  Ingestion

    d.  Inhalation

2.  Midnight dumpers have been leaving toxic waste in an illegal
    dumpsite in the woods. The soil beneath the waste is highly porous.
    A nearby house has a well, which taps into this porous soil. Based
    on this limited information, through what ***medium*** is the well
    most likely to become contaminated?

    a.  Soil

    b.  Air

    c.  Surface water

    d.  Groundwater

3.  You spill a toxic substance; you cough, and your eyes immediately
    begin to water. You are experiencing what type of exposure?

    a.  Acute

    b.  Chronic

4.  You have just learned that a chemical you work with is a
    ***teratogen***. What effect does this chemical have?

    a.  Increases the risk of cancer

    b.  Increases the risk of physical defects in a developing embryo

    c.  Causes a permanent change in the genetic material (DNA)

    d.  Irritates the lining of the throat

5.  When you are about to have a serious operation, the anesthetist is
    likely to use a substance which is:

    a.  A CNS (central nervous system) depressant

    b.  An irritant

    c.  An asphyxiant

    d.  A corrosive

6.  Which of these groups would be likely to have the ***most extreme
    reaction*** to most types of toxic exposure?

    a.  Adult women

    b.  Adult men

    c.  Children

    d.  Adolescents

7.  Groundwater is ***least*** likely to be contaminated under what
    circumstances?

    a.  The waste is confined by an aged underground storage tank

    b.  The groundwater is protected by a relatively impermeable
        protective layer, such as clay

    c.  The contaminants are persistent

    d.  The contaminants are highly soluble