Unit 6: Risk, Exposure, and Health // Section 6: Other Risks
Environmental contaminants cause many harmful effects in addition to cancer, such as toxicity, birth defects, reduced immune system function, and damage to other organs and physical systems. For noncarcinogens, researchers assume that a threshold exists below which no harmful effects are likely to occur in humans. To quantify these values, scientists first seek to identify the so-called no observable adverse effects level (NOAEL), which is the highest exposure among all available studies at which no toxic effect was observed. Next they divide the NOAEL by one or more uncertainty factors, typically ranging from 10 to 1,000, based on the quality of the data that was used to measure the NOAEL and on how close the NOAEL is to estimated human exposures.
From these calculations, EPA sets reference doses for ingestion and reference concentrations for inhalation that represent levels at which humans can be exposed to chemicals for specific periods of time without suffering adverse health effects. These limits are fairly conservative because they incorporate uncertainty factors and assume that people may be exposed daily or constantly throughout their lives. Box 1 shows EPA's core health assessment figures for noncarcinogenic effects of paraquat, a widely-used and highly toxic herbicide.
Box 1. Chronic health hazard assessment of paraquat for noncarcinogenic effects
Critical effect: chronic pneumonitis (lung inflammation)
No observed effects level (NOEL): 0.45 mg/kg-day
Uncertainty factor: 100 to account for inter- and intraspecies difference in extrapolating from laboratory animals (dogs) to humans
Reference dose for oral exposure: 4.5 x 10-3 mg/kg-day
[Source: EPA, Integated Risk Information System, http://cfpub.epa.gov/iris/quickview.cfm?substance_nmbr=0183]
Regulators also set limits for specific types of exposures. For example, the EPA establishes guidelines for pesticide residues in food, and the Agency for Toxic Substances and Disease Registry establishes minimal risk levels (MRLs) for acute, intermediate, and chronic exposure to contaminants at hazardous waste sites.
The EPA's peer-reviewed assessments of human health effects (both cancer and non-cancer) from exposure to chemicals are available through the agency's Integrated Risk Information System (IRIS) (footnote 17). These reports include descriptive and quantitative information on specific chemicals that cause cancer and other chronic health effects. Analysts can use this information along with exposure information to characterize public health risks from specific chemicals in specific situations and to design risk management programs.
The state of California has developed a similar list in compliance with Proposition 65, a 1986 ballot measure that required the state to publish a list of chemicals known to cause cancer, birth defects, or reproductive harm (footnote 18). Chemicals can be listed in three ways: if they are shown to cause cancer, birth defects, or reproductive harm by either of two state expert committees; if they are so identified by EPA, certain other U.S. regulatory agencies, or the International Agency for Research on Cancer; or if a state or federal agency requires them to be labeled as causing these effects (substances in this category are mainly prescription drugs).
Companies that do business in California must provide "clear and reasonable" warning before knowingly and deliberately exposing anyone to a listed chemical, unless exposure is low enough to pose no significant health risks. They also are barred from discharging listed chemicals into drinking water sources. The intent of Proposition 65 is to increase awareness about the effects of exposure to listed chemicals, enable Californians to reduce their exposure, and give manufacturers an incentive to find substitutes for listed chemicals. The law has led to removal of many toxic substances from commerce, including faucets and tableware that contained lead.