May 21, 2013

Control of Air Pollution From Motor Vehicles: Tier 3 Motor Vehicle Emission and Fuel Standards

The Environmental Protection Agency (EPA) is proposing new vehicle emissions standards and a reduction in the sulfur content legally permitted in gasoline. According to the EPA, these new standards will generate improvements to the environment and to public health, primarily by lowering the emissions of pollutants, like particulate matter (PM) and ozone. The EPA believes these pollutants contribute to increases in human mortality and other health problems.

We find that the EPA has failed to acknowledge the high degree of uncertainty surrounding its estimates of benefits from this regulation. A growing literature calls into question the causal link between the total concentration of ambient particulate matter and mortality levels, especially at the low doses that exist today in many parts of the United States. Additionally, the EPA failed to consider other adverse effects of its rules, such as impacts on low-income individuals and adverse employment effects that were overlooked in the EPA’s scant employment impact analysis.


While the EPA should be commended for pursuing the laudable goal of a cleaner environment and improvements in public health, there are several reasons to be skeptical of the benefits the EPA has claimed will result from this regulation.

First, over 50 percent of the quantified benefits from the regulation are due to reductions in total particulate matter. However, a growing literature raises doubts about the causal link between ambient PM2.5 levels and increases in mortality. Additionally, the EPA regulates both PM and Ozone under the National Ambient Air Quality Standards (NAAQS), which makes the proposed regulation an indirect, and perhaps impractical, way to achieve the EPA’s objectives.

Additionally, there is a high degree of uncertainty, which the EPA itself acknowledges, surrounding the EPA’s benefits estimates. For example, the EPA describes criticisms related to its uncertainty analysis made by the National Resource Council in a 2002 report.[1] Despite the EPA’s acknowledgement of these criticisms, the EPA continues to evaluate uncertainty in a similar manner.[2] The EPA also acknowledges uncertainty with regards to its benefits analysis, including these points from table 8-16 of the Regulatory Impact Analysis for the rule:

1. The extrapolation of effect estimates is beyond the range of ozone or PM concentrations observed in the source epidemiological study.

2. Direct causal agents within the complex mixture of PM have not been identified.

Each point is important. First, the EPA states that its estimates go beyond those confirmed in the epidemiological study upon which it bases its findings. This means benefits of the regulation are based primarily upon model selection, not empirical evidence. The EPA assumes a linear-dose response down to the origin, resulting in large benefits estimates. Selecting another model, such as a threshold- or hormetic-dose response at low doses, would produce vastly lower benefits estimates. Recent academic literature has suggested there may be reason to believe PM exhibits a hormetic-dose response at low doses.[3]

Next, the EPA fails to address whether the concentration of total particulate mass or the composition of those particulates are the cause of the health effects found in the cited studies. Moreover, because the composition of rural particulates is different from urban particulates, the health effects are likely to be different than those estimated.[4] In order to provide a causal link, the EPA should be able to determine which components of particulate matter are the sources of the higher morbidity and mortality rates. For instance, Bell finds that higher concentrations of PM2.5 Nickel are associated with higher rates of cardiovascular or respiratory hospitalizations.[5]

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