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The dictionary defines risk as the "possibility of suffering harm or loss; danger." Scientists use the term risk when assessing potential human health threats from exposure to chemicals or pollutants in the environment. Risk is equal to a person's exposure multiplied by the toxicity of the chemical.
Exposure is a combination of the concentration of the chemical a person is exposed to and the length of time they are exposed to that chemical. Toxicity is a measure of the degree to which something is poisonous and is often expressed as a dose-response relationship.
Almost every substance is toxic at some dosage. That is, anything in a large enough quantity can be poisonous. For example, an adult ingesting half a cup (400 grams) of salt can be fatal. Even water, if consumed in large enough quantities, can be fatal.
The above diagram illustrates how people are exposed to chemicals and how exposure to these chemicals can affect human health. This information is used in assessing potential risk, or the possibility of suffering certain effects. As exposure to or toxicity of a chemical increases, human health risks also increase.
Some common data gathered to study risk include:
How much risk a chemical presents can also depend on how the body is exposed to the chemical. IDEM focuses on three methods of chemical exposure:
Once a pollutant enters a person's body it may:
In the blood, chemicals can be carried to all parts of the body. As chemicals move through the body, they may undergo chemical changes and become less or more toxic.
Exposure to chemicals in the environment can result in health effects that range from relatively minor to severe. They include:
If IDEM or another agency believes that land, air or water contains chemicals that are harmful to people, animal life or the environment, one course of action is to conduct a risk assessment.
Reports from local citizens about health symptoms and suspicious odors, dust and other contaminants can bring high-risk situations to IDEM's attention. Also, several national studies use information reported from businesses and industries to discover high risk areas.
To conduct a risk assessment, the agency takes field samples, measures the chemical levels in the samples and determines if the chemical levels pose a risk to human health.
Though risk assessment projects use different methods and techniques, all go through four basic steps to characterize risk.
Review key research to identify any potential health problems that a chemical can cause.
Determine the amount, duration and pattern of exposure to the chemical.
Estimate of toxicity (i.e. how much of the chemical it would take to cause varying degrees of health effects that could lead to illnesses).
Assess the risk for the chemical to cause cancer or other illnesses in the general population.
Non-cancerous effects may occur when chemical exposure reaches the dose-response level, or threshold. Thresholds are different for each chemical and vary depending on how much of the chemical is present and how long the exposure lasts.
Some health problems occur very soon after a person is exposed to a chemical. Symptoms or illnesses occurring from short-term exposure are called acute effects. The immediate effects may be minor (i.e. watery eyes, rash, throat irritation) and may go away once a person is no longer exposed to the chemical. However, some acute exposures may cause serious problems, such as damage to the lungs.
Other health problems may not appear unless a person is exposed to a chemical for many years. These effects are a result of long-term exposure and are called chronic effects. The threshold for chronic effects is usually much lower than the threshold for acute effects since chemicals can build up in the body over time. Cancer is an example of a delayed health problem that could be a result of long-term exposure to a chemical.
However, cancer-causing chemicals affect the body differently than non-cancer causing chemicals. Risk assessors assume that cancer risks from chemical exposures do not have a threshold. They assume that any amount of exposure will increase the probability of cancer development over 70 years (considered a lifetime) to determine risk.
The risk associated with the potential to develop cancer after exposure to chemicals is often expressed as a probability or a fraction in a range from zero to one (0.0-1.0). A zero chance would mean there is no chance of developing cancer and a one chance would mean there is absolute certainty that one will develop cancer. Usually the numbers are very small and shown in fractions of one million or fractions of one hundred thousand. The table below shows the various ways these fractions may be presented.
|Number and how it reads in various scientific notations|
|1/10||0.1||1x10-1||1.0E-1||One in ten|
|1/100||0.01||1x10-2||1.0E-2||One in a hundred|
|1/1,000||0.001||1x10-3||1.0E-3||One in a thousand|
|1/10,000||0.0001||1x10-4||1.0E-4||One in ten thousand|
|1/100,000||0.00001||1x10-5||1.0E-5||One in a hundred thousand|
|1/1,000,000||0.000001||1x10-6||1.0E-6||One in a million|
|1/1,000,000,000||0.000000001||1x10-9||1.0E-9||One in a billion|
The numbers above should not be confused with the concentrations of chemicals, which are used for calculating exposure and will be explained further below.
The probability of cancer development in risk assessments does not represent a person's total cancer risk. This cannot be estimated from risk assessments since the probability of cancer development depends on many individual factors, such as genetic history, diet and personal habits (i.e. smoking, exercise). For example, a chemical in the air may pose a one in a million risk of cancer to a person (i.e., if one million people breathed that air for 70 years, most likely no more than one of them would develop cancer from that chemical exposure), although hundreds of thousands will develop cancer from various other causes.
Risk characterizations use statistics to determine which chemicals pose the highest risks. The numerical results represent an estimated probability, similar to the probability of a coin landing heads up (a one in two chance). While the numbers help scientists and regulators, the size and scale are often hard to relate to every day life. Below are some statistics based on recording past incidences. While these numbers are slightly different from risk estimates, they add perspective to the scale of the risk numbers.
Personal habits and other factors can influence a person's actual risk. For example, you can see above that the statistical risk of dying in a car accident in a lifetime is 1 in 85 (12,000 in 1,000,000). A person commuting a long way to work, who would be driving more than the average driver, would expect their actual risk of dying in a car accident to be higher than 1 in 85, on the other hand a person who rides the subway to work instead of driving would have a lower risk. In the same way, actual risks from chemical exposure are hard to predict exactly given the number of variables used in the assessments.
One example of how personal risk is recognized and managed is the distribution of flu vaccines during a vaccine shortage. Statistically, children and the elderly have the greatest risk of death from contracting the flu, so vaccines are reserved for them. Risk assessments work to identify similar risk hazards to help governments, businesses and citizens prioritize efforts to reduce overall risk.
Estimating risk is a complicated process. It involves taking data and multiplying it by a risk factor to determine the likelihood of a health effect. The data must be collected using methods approved for risk assessments and must be verified. The agency takes field samples, measures the chemical levels in the samples and determines if the chemical levels pose a risk to human health. The data is then compared to acute thresholds, chronic thresholds and the probability of cancer development.
The measurements used to determine risk can be very confusing because they are made in unfamiliar quantities. For example, when we speak of exposure, measurements such as micrograms per cubic meter (µg/m3) are sometimes used. Micrograms per cubic meter describes the concentrations of chemicals in the environment. Concentrations of chemicals in air are typically measured in units of the mass of chemical (milligrams, micrograms) per volume of air (cubic meter). However, concentrations may also be expressed as parts per million (ppm) or parts per billion (ppb) by applying a conversion factor. The formula to convert a concentration from micrograms per meter cubed to parts per billion is based on the molecular weight of the chemical and is different for each chemical. Also, atmospheric temperature and pressure affect the calculation. Most of the time a standard atmospheric temperature (25 °C) and atmospheric pressure (1 atm) are used.
For example, in air, a number of 5 ppb would mean that there are 5 parts of the chemical for every billion parts of everything else in the air. 5 ppb does not mean the same thing as 5 µg/m3.
The concentrations (ppb or µg/m3) are used to determine exposure concentrations. These concentrations are then used to calculate risk estimates for both cancer and non-cancer effects.
Due to variables and unknown factors, it is impossible to accurately estimate risk. When available, data from human exposure is used to estimate risk. In many cases, however, scientists do not have all of the details on actual exposures or how a chemical harms human health. Dose-response factors, or toxicity, are based on studies done on animals or cells, not actual human exposure. Also, risk estimations sometimes use computer models to calculate the effects of chemical exposure when actual studies are not available.
Results between different studies are not always consistent. When this happens, IDEM chooses the most accurate and health-protective study. This health-protective assumption prevents the agency from underestimating risk.
Risk assessments can lead to companies emitting fewer chemicals or using chemicals that are less toxic. Governments use risk assessments to encourage businesses to maintain safe chemical levels in the environment and to identify areas where further information and regulation is needed. This includes restrictions of ongoing emissions and discharges as well as clean-up of contaminated land and water bodies.
In the same way, risk assessments can also help governments identify sources of emissions, such as cars and diesel trucks, and take action to reduce the release of these chemicals. A simple example is vehicles. For a local impact, changing a traffic light's timing may reduce vehicle idling and toxic emissions in a neighborhood. A complex, national example is altering the required control technology for certain categories of vehicles. This type of emission reduction can be applied to vehicles, industries, small businesses, governments and even citizens and will be effective on a much larger scale.
Risk assessments only explain one part of an individual's total risk. It is the agency's goal to arm the public with information about risks from environmental exposures to chemicals so they can make informed decisions about their health. However, assessments leave out risk that comes from the habits and choices in an individual's lifestyle. Household cleaners may contain chemicals that increase acute, chronic and cancer risk. Smoking dramatically increases cancer risk. On the other hand, exercise can decrease cancer risk. When studying environmental risk, remember that it makes up only one part of the total risk in our lives.