It should be noted that risk assessment is an iterative process having a number of assumptions. Health risks may have been overestimated due to the fact that the risks calculated based on
the chemical concentrations measured in a short time were compared to specified risks developed based on the toxicological data established for exposures over a lifespan. With respects to underestimating potential health risks, only the concentrations of BTEX among other LFG constituents (i.e., hydrogen sulfide, ammonia, and other NMOCs) were analyzed in this study. Exposures occur only if the landfill is producing harmful levels of gases and if the gases are migrating from landfills and reaching people. LFG constituents are typically found in ambient air at low concentrations unlikely to cause adverse health effects [50]. However, whether landfill emissions pose a health hazard depends on the chemical concentrations to which people are being exposed and the duration of the exposure. ATSDR conducted a health study of communities near the Fresh Kills Municipal Landfill in New York and concluded that odor producing chemicals (i.e., hydrogen sulfide and ammonia) are not associated with landfill emissions. The odors associated with these chemicals can, however, cause acute effects, such as nausea and headaches. Acute effects from other chemicals found in landfill gas are usually produced only when an individual is exposed at relatively high concentrations. On the other hand, health effects of NMOCs need to be considered on a chemical-by-chemical basis. In general, levels of individual landfill gases in ambient air are not likely to reach harmful levels. However, the potential cumulative effects of being exposed to low levels of the types of NMOCs emitted from landfills should be evaluated. The uncertainties in toxicological indices also contributed to overestimating or underestimating risks. The pharmacokinetics of testing animal species with high doses may not be exactly the same as low doses in humans. Risks may have also been underestimated because only inhalation exposures were considered although exposure by ingestion or dermal absorption is usually much lower. Finally, the risk calculation is an exercise in applied probability of extremely rare events. This introduces an inherent conservatism which often results in assessing scenarios that will never be experienced.