Fundamentally, the problems in this and all quantiative risk assessments are related to the quality of the available data and our need to extrapolate either from animals to humans or from high to low exposure levels. 3 Subsequent research has indicated that the response in rats may be related to overload of lung clearance mechanisms in the rat. The authors of these studies have suggested that this mechanism is unique to the rat and that these data cannot be used for predicting human risk. We should first recognize that the problem is not so much with risk assessment as it is with our complicated regulatory decision-making process. Blaming risk assessment for our problems is analogous to killing the messenger when you don't like or fully believe the message.
Risk assessment is a vital and irreplaceable tool for informing public health decision making.
However, expecting risk assessment to provide definitive characterizations of human risk is naive and a recipe for procrastination in making important decisions for protecting the public's health. There will always be substantial uncertainties in risk assessments and substantial room for scientific debate about whether there is a risk and how best to quantify it. While additional research may reduce the uncertainties in risk assessment for diesel exhaust exposure, it will not totally eliminate them. Although these studies were designed with the intent of clarifying the quantitative relationship between diesel exhaust and cancer, grounds for scientific debate will still remain. Given the large number of previous diesel exhaust studies, it is unclear whether any single new study will significantly alter our position on this issue.