Background and Understanding

Two pieces of landmark legislation – the 1969 National Environmental Policy Act (NEPA) and the 1970 Clean Air Act – provide guidance on air quality monitoring at all levels of government. NEPA established the procedural policy and goals for the protection, maintenance, and enhancement of the environment. This regulation requires that agencies using federal funds must undergo a process (the so called NEPA process) to consider the environmental consequences of any proposed action with the potential to significantly affect the quality of the human environment, as well as to identify related mitigation measures and consult with, and incorporate comments from, other parties including the public. In addition, the Clean Air Act and its subsequent amendments, in particular the Clean Air Act Amendment (CAAA) of 1990, provide guidelines on air pollutants.

Since the designation of 188 air toxic pollutants as Hazardous Air Pollutants (HAPs) by the CAAA of 1990, several studies have identified the effects that toxic substances have on the human population and the environment. In 2001, the U.S. Environmental Protection Agency (EPA) identified a list of 21 HAPs as Mobile Source Air Toxic (MSAT) substances, with a subset of seven compounds prioritized for mitigation, given their potential for adverse health effects. They are acetaldehyde, acrolein, benzene, 1,3-butadiene, formaldehyde, naphthalene, and polycyclic organic matter. Indeed, research on air toxic emissions has linked emissions from mobile sources to cancer risks. For example, the Multiple Air Toxic Exposure Study-II (MATES II) study, and then the MATES III found that approximately 90% of the cancer risk from toxic air contaminants may be related to mobile sources, and these reports assigned high rates (70% and 84%, respectively) of the total risk from such sources to diesel particulate matter (DPM). Similarly, other studies have found that proximity to highways is associated to adverse health effects, including research conducted by a member of our team.

In addition to the existing assessment of air pollutants such as NOx, CO, etc, recently, the EPA and environmental organizations have demanded that, in light of associated risks, the emissions from mobile sources should not be ignored when using federal funding. To help facilitate the MSATs analyses in the NEPA documents of State DOTs, the Federal Highway Administration (FHWA) issued two interim guidance reports, first in 2006 and an update in 2009. The guidance provides three levels of MSATs analyses – no analysis, qualitative analysis, and quantitative analysis - that State DOTs can choose depending on the significance of MSATs impacts on the public. Indeed, some states utilize the FHWA guidance with or without modifications, and at least two states, California and Colorado, have been developing their own procedures tailored to their own contexts. However, there still exists a lot of confusion on specific threshold, analytical methodology, and potential mitigation measures. As a result, a number of transportation agencies have been straining to meet the requirements for adequate MSAT analyses in NEPA documents, or are being sued for failure to meet such requirements. This is, in part, because, unlike the National Ambient Air Quality Standard (NAAQSO) for such pollutants as CO, NOx, PM10, etc., there is no agreement on the threshold values or criteria for MSATs, which may assist state DOTs in evaluating proposed actions. At the same time, the FHWA has expressed its concern that the tools and techniques for assessing project-specific health impacts from MSATs are not well advanced. That is, the lack of agreement about appropriate dispersion models for assessing project level health risk of air toxics is of concern to several state DOTs.

Given such legal and operational challenges, proposed transportation projects are being delayed, thus contributing to a backlog in the delivery of important transportation improvements. Therefore, agencies would benefit from having clear guidelines to help them implement a “triage” process that will facilitate the determination of determination of which projects merit further evaluation, and which ones may proceed without additional appraisal.