| Risk
Assessment
| Exercise: |
List
the major aspects that determine a person's health. Rank these
by importance, taking into account the most important factors.
What do you learn from this exercise about the way in which
the natural environment affects our health? |
|
Risk assessment
is the main method used by agencies such as the EPA to estimate the environmental
impact of pollution, to compare different alternatives for managing pollution,
and to set regulatory goals and guidelines that industry and municipalities
must meet. Why? Because you can calculate a number known as a risk, and
numbers are easier to work with when doing comparisons. For example, we
said the wastewater treatment plant did not get rid of the pollution,
but merely altered its form and where it ended up. Is that good or bad?
One way to answer that question is to determine the total risk before
treatment (discharge direct to water body) versus the total risk after
treatment including the risk from air emissions, sludge disposal, disinfectant
byproducts in the water body, and treated wastewater disposal.
That brings
up another question—calculate the risk to what? Most of the risk
assessments performed currently are for the risk to human health, however
you could think of risk in terms of risk to a particular species in the
ecosystem/biosphere, etc. The general framework for a human health risk
assessment is presented below since this is what we currently know the
most about.
| |
| Figure
5: Risk Mangagement Scheme |
Figure 5
shows the general scheme involved in assessing and managing risk. The
first step is identifying the concerns and magnitude to see what type
of hazard is posed by an exposure event that poses risk. The information
about biological effects of the agent may already be available—or,
as in the case of many new synthetic chemicals, and microbes, it may not
be. These may be a whole scientific project that may be needed to obtain
this information.
Rather
than describing the steps in theory, the next section goes through a specific
example of how a risk assessment calculation is done.
Sample
Risk Assessment - Groundwater Contamination
At many industrial sites, chemicals are stored in underground storage
tanks (USTs). At a particular site, an UST containing chemical X is known
to be leaking. An existing monitoring well close to the tank showed that
chemical X has reached the groundwater below. A neighboring town gets
part of its water supply from the contaminated aquifer and part from another
source. How would you determine if there is a risk to the town? List the
various steps you need to do and what information you must have to do
a quantitative risk assessment.
The general
EPA framework for quantitative risk assessment is as follows:
1. Hazard Identification
This is the gathering of existing information about a particular chemical's
risk:
- What
chemicals was subject exposed to?
- Can the
chemicals have adverse health effects?
- What
are those effects - mutagenesis (mutations to genetic material), carcinogens
(cancer causing), teratogens (birth defects), acutely toxic (short-term
illnesses and death)?
- What
are the various pathways by which a chemical could affect subject population
- ingestion, inhalation, dermal?
Note
that the effects are determined by two methods: toxicity testing with
animals and epidemiological studies of previous exposures. However, there
are problems with both methods including:
| a) |
toxicity
testing with animals: expose animals (rats, mice, guinea pigs) to
varying (but large) doses of chemical and monitor mutagen occurrence,
tumors in organs, and death rates. Problems are 1) extrapolate (often
linearly) the effects caused by large doses (needed so can see effects
quickly) to low doses that humans are more typically exposed to,
2) humans have different metabolic rates than the animals tested. |
| |
|
| b) |
epidemiological
studies: study (over the long term) the effects on individuals who
have been inadvertently exposed to a chemical and comparing their
health with the general population. Problems are 1) since it is
not a controlled study there are many confounding factors such as
diet, smoking habits, genetics, etc., 2) takes a fairly long time
to obtain conclusive results since you typically need to monitor
subjects over a lifetime |
2.
Exposure Assessment
To have a risk, the subject population has to be exposed to the
chemical.
- What
are the possible pathways? (e.g., dermal, ingestion, inhalation of water
air, soil, and/or food)
- How much
contact is there? (How much chemical is in environment, what happens
to the chemical along a pathway (degradation), how much chemical reaches
an exposed person?)
- How much
chemical gets into a person's body (dose), considering body weight,
length of exposure, bioconcentration, etc., in units of mg/kg/day?
The EPA
typically assumes the subject is a 70 kg male who lives 70 years in the
same location. Problems with this assumption are many including the role
of ethnicity, sex, and age.
3.
Dose-Response Assessment
This includes taking what is known about the chemical scientifically and
accounting for the actual dose a person is exposed to and determines the
risk.
Toxicologists
develop relationships between toxicity testing information and risk that
are called dose-response curves. These curves are actually linear - for
carcinogens they go through 0 and for acutely toxic chemicals there is
a threshold dose. This data exists for several chemicals and can be obtained
from the National Institute of Health, IRIS database, EPA. For carcinogens
the relationship is known as the potency factor (mg/KG/day)-1. For acute
toxics the relationship is known as LOEF, NOEL, RfD.
4.
Risk Characterization
This is where you take all the information (qualitative and quantitative)
from above and determine if there is a risk that needs to be managed.
Some suggestions are as follows:
- Calculate
individual risk
- Calculate
population risk - typically EPA sets maximum risk at 1 in a million
for most regulations
- Compare
risk to existing death rates, etc.
- Compare
risk to regulated standards
- Consider
uncertainty, problems with data, conservatism, etc.
Let’s
decide if there is a significant risk to the community. X concentration
in the groundwater below UST is 300 mg/L. Groundwater flows at 1 ft/day
and the supply well is 1 mile from the UST. The pump rate is 75,000 gpd.
The groundwater is mixed with uncontaminated water for a total supply
of 1 Mgpd for 50,000 people. The chemical decays with a half-life of 10
years assuming a first order decay rate, and the rate that X flows is
half the rate of the groundwater. Assume no dispersion. The potency factor
for X is .02 (mg/Kg/day-1). Note that in the US the cancer death rate
is 193/100,000.
Hazard
Identification:
X is a carcinogen
Cpf = 0.02 (mg/kg-day)-1
Pathways = ingestion, dermal, if volatile then inhalation
Exposure
Assessment:
only consider ingestion
chemical decays and absorbs to soil so lower concentration at well
velocity is 0.5 ft/day due to sorption
dose has to account for amount of water per day drank - EPA scenario assumes
2 L per day for a 70 kg male over 70 years
Time to get to well = 1mile/0.5 ft per day = 5280/0.5 = 10,560 days
k = 0.693/(10*365) = 1.9 * 10-4 day-1 (C= C0e-kt)
C = 300 e-(1.9*10-4*10560) = 40 ug/L
In drinking water conc = (75000*40+0)/ 1000000 = 3 ug/L
Daily dose = 3*2/70 = 85.7 * 10-6 mg/kg/day
Dose
Response Assessment:
risk = 0.02*85.7*10-6 = 1.7 * 10-6 or approximately 2 in a million
Hazard
Charactarization:
lifetime individual risk = 2 in a million
EPA standard typically 1 in a million
Population risk = (50000*1.7)/(70*1000000) = 0.001 people will die annually
of cancer in town
existing
US cancer death rate is 193 in 100000 so for 50000 = 96.5 people per year
conservative with lots of uncertainty
PREV
| NEXT |
