October-December 1999 Issue


Regulating Hazardous Air Pollutants
in Urban Areas: Recommendations from
Energy Laboratory Symposium

n July 1999, an Energy Laboratory symposium of scientists, regulators, and industry and public interest representatives examined the scientific challenges posed by the US Environmental Protection Agency's new Integrated Urban Air Toxics Strategy. This strategy describes EPA's plans for substantially reducing public health risks in urban areas from air toxics, or "hazardous air pollutants" (HAPs), a wide-ranging group of emissions that cause cancer or other health problems and are emitted by cars, smokestacks, dry cleaners, auto body shops, and many other sources. ("Criteria" pollutants such as ozone and sulfur dioxide are regulated separately.) Symposium participants recommended continuing research on sources of HAP emissions; on processes whereby HAPs change in the atmosphere; on routes by which HAPs enter the human body; and on health effects, including the effects of exposure to multiple HAPs and of variations among individuals. Other recommendations included revising EPA's plan to monitor many HAPs in many locations. Monitoring should instead focus on critical HAPs in selected areas and on sources that are controllable. More attention should be given to indoor air pollution, the primary health risk for many people. Work to identify the dangerous constituents in diesel exhaust should continue, but tighter restrictions on overall diesel emissions are probably warranted now. High priority should be placed on controlling mercury emissions, especially from electric power plants. Finally, EPA should communicate better, both with the general public and with state officials, who have the most experience with data and programs particular to their regions.

For decades, attention has focused on controlling criteria air pollutants such as nitrogen oxides, sulfur oxides, ozone, and particulates. But another class of air pollutants--the air toxics, or hazardous air pollutants (HAPs)--is coming under increasing scrutiny. Among the HAPs are benzene, mercury, formaldehyde, and a host of less-familiar-sounding chemicals that are known or suspected to cause cancer or other serious health effects. During the past decade, the Environmental Protection Agency (EPA) has issued regulations that have substantially reduced emissions of HAPs from cars, trucks, fuels, and industries such as chemical plants and oil refineries. However, concern continues to focus on urban areas, where populations are high, emissions sources are many and varied, and emissions from even small sources may combine to have unexpectedly significant health impacts.

In mid-1999, EPA released its Integrated Urban Air Toxics Strategy, a plan that complements existing air toxics regulatory programs and focuses specifically on protecting human health in urban areas, as instructed in the Clean Air Act Amendments (CAAA) of 1990. The strategy is an unprecedented move for EPA: instead of presenting a set of rules and regulations, it describes objectives, priorities, and schedules for ongoing actions that will reduce public health risks from HAPs in urban areas. Included are steps to improve EPA's understanding of health risks posed by urban air toxics, programs to determine what pollutants are present, and initiatives to address specific pollutants and specific community risks. Finally, the strategy calls for education and outreach efforts to inform stakeholders and the public about the strategy and to get input into designing programs to implement it.

In July 1999, the Energy Laboratory hosted a two-day symposium to discuss scientific aspects of the Integrated Urban Air Toxics Strategy. The symposium was part of a series of annual conferences begun in 1993 for the purpose of informing decisionmakers about scientific aspects of important air pollution issues and improving communication between the scientific and regulatory communities. Participation at the 1999 symposium was limited to invited members of the scientific, regulatory, industry, and public interest communities. Together, this group examined the requirements and challenges of the air toxics strategy and proposed specific recommendations on how best to implement it.

Included in the strategy are lists of 33 HAPs and 29 types of sources that EPA regulates or has plans to regulate. While those lists were based on extensive analyses of available data, further studies are needed to confirm whether the existing and planned regulations are sufficiently strict and whether additional targets should be added. Presentations and discussions at the symposium outlined the scientific challenges involved in that task. The figure below, presented at the symposium, shows the components that must be considered and serves as a basis for understanding the difficulties involved at each step.

Sources. HAPs may come from "point" sources (typically smokestacks, which emit large quantities of HAPs), "mobile" sources (all types of vehicles), and "area" sources (dry cleaners, gas stations, and other small but ubiquitous sources that together contribute significant quantities of urban emissions). The Integrated Urban Air Toxics Strategy looks collectively at all three types of sources but focuses particularly on area sources because of the regulatory challenge they pose. A given HAP may originate from many types of sources, both outdoors and indoors; and a single source may emit many types of HAPs. In addition, some HAPs form in the atmosphere from chemical reactions of other substances, making them particularly hard to control.

Ambient Concentrations. Predicting the chemical make-up of the atmosphere is difficult because many emitted HAPs transform in the atmosphere. They may degrade into nontoxic components or react to form products that are more hazardous than the emitted substances. Most atmospheric "transformation products" remain unidentified. Some HAPs (for example, mercury) ultimately settle onto the earth's surface and can enter the food chain via non-inhalation routes. And some HAPs occur indoors more than outdoors. Programs to monitor ambient concentrations can provide important insights, but questions abound. What should we measure and where? Should we measure where sources are abundant or where populations are high? And how do we ensure that consistent monitoring techniques are used in all locations?

Exposures. Most human exposures to HAPs occur through breathing, but eating and touching are also important exposure routes. The level of exposure depends not only on ambient pollutant concentrations but also on personal behavior. A person who pumps gasoline will have higher exposures to escaping benzene vapors than will a person who just stops to buy gasoline. And a person who spends little time outdoors will be exposed mostly to indoor pollutants.

Health Effects. Primary concern focuses on cancer-causing pollutants, but HAPs are also associated with neurological, reproductive, and developmental disorders. Establishing whether an air toxic threatens health is difficult. One problem is variability in the population. One person may be unharmed by a particular HAP, while another may react to low levels of the same substance. There may be "cumulative" effects from exposure to several substances that cause the same type of damage. And there may be "synergistic" effects. Two substances may be minimally hazardous by themselves. But when both are in the body, one substance may prime the body to be more sensitive to the other. An added problem for researchers is how to quantify health effects when they occur. Methods exist for calculating the carcinogenicity of a substance, but quantifying and comparing the seriousness of widely varying non-cancer effects remain difficult.

Emissions Control Regulations. Clearly, the process by which emissions from specific sources ultimately cause health effects is extraordinarily complex. A better understanding of the various steps in that process and how they are linked would permit EPA to set more effective and appropriate emissions control regulations back at the source.

Presentations at the symposium showed that researchers still need to develop data, to understand processes and mechanisms, and to address uncertainties at each step in order to design better control strategies for various sources. Major challenges include defining a prioritized research agenda and designing and implementing appropriate regulatory actions in the near term, while knowledge is still evolving. Resources--both financial and human--are limited, so every research task or policy undertaken has an implicit trade-off. Undertaking one activity precludes undertaking another one. With that limitation in mind, symposium participants made several recommendations.

Most notable was their criticism of EPA's proposed emissions monitoring program, which calls for all 50 states to monitor HAPS. The participants recommended that EPA not try to measure everything everywhere but instead carefully plan a monitoring program with clear objectives and priorities. The first task should be to analyze the abundant data that have already been collected but not yet thoroughly examined. The monitoring program should be designed to fill the gaps and should be based not on political pressures but on scientific and regulatory objectives. The focus should be on compounds most relevant to health, on sources where emissions control is feasible, and on locations carefully selected to contribute to scientific understanding. In addition, EPA should begin taking a holistic approach to monitoring. For instance, under EPA's program to reduce criteria pollutants, states are monitoring sources of particulate emissions. Many HAPs come from the same sources, but there is as yet no provision for coordinating the particulate and HAPs monitoring programs--a step that would increase efficiency and reduce cost.

Participants generally agreed that insufficient attention is given to indoor air pollution. Almost all monitoring takes place outdoors. Yet there are significant indoor sources of HAPs; concentrations of many HAPs are higher indoors than outdoors; and people spend a lot of time indoors. (In fact, many people spend less than one hour outside each day.) Participants therefore recommended that EPA revise its monitoring program to include analysis of indoor air quality and indoor sources of contaminants.

Another area that needs more attention is emissions from the diesel engine. Considerable evidence already suggests that diesel exhaust causes adverse health effects. But diesel exhaust is a complex mixture of particles, gases, and vapors; and it is not yet clear which constituents threaten health. Symposium participants agreed that more research is needed to identify those constituents and to understand their impacts.

However, they disagreed as to what regulatory steps should be taken now. EPA has described plans to tighten regulations on diesel exhaust. The new regulations would continue EPA's practice of testing and regulating diesel exhaust as a single entity--an unusual approach, as most regulations focus on individual compounds rather than mixtures. Some participants recommended that EPA wait until the offending constituents are identified so that targeted control strategies can be developed. They also pointed out that existing regulations are already substantially reducing emissions from diesels and that taking further (perhaps unwarranted) action could threaten the future viability of this fuel-efficient engine technology. Other participants argued that testing and regulating real-life mixtures is "good regulatory science." In their view, EPA should take further regulatory action now; and scientists should continue to investigate the health impacts of diesel exhaust.

In contrast, the need to control mercury emissions was unquestioned. Mercury is the only HAP on EPA's list that threatens people primarily by ingestion rather than inhalation. It is a powerful neurotoxin that is neither created nor destroyed by combustion or atmospheric reactions. Airborne mercury settles on land or water and ends up highly concentrated in plants and animals that humans eat. Forty states have issued fish consumption advisories because of mercury contamination of freshwater and marine fish.

EPA considers mercury from coal-fired power plants the HAP of greatest potential concern. Mercury occurs as a vapor in flue gases at the part-per-billion level, so control is difficult. Also, because concentrations are so low, mercury does not appear in official reports of toxics releases filed by utilities. Symposium participants supported EPA's plans to require more stringent reporting of mercury emissions. Electric power experts described control strategies now being developed but emphasized that many engineering aspects are not yet well understood. Control of mercury from power plants may be achieved within three to five years, but the task will be difficult.

Medical and municipal waste combustors are other major sources of mercury. Here the news is more encouraging. Hospitals continue to rely on mercury in thermometers, switches, and other devices that sense small temperature changes. Nevertheless, a mercury-control program in New Jersey has eliminated up to 99% of the mercury occurring in exhaust from medical waste incinerators, largely by focusing on hospitals' use of mercury-containing devices. The program has also substantially reduced mercury emissions from municipal waste combustors. Symposium participants recommended that EPA take more aggressive steps to get mercury out of consumer products.

Participants also advised EPA to work harder at communicating with the public. People have pragmatic concerns such as whether eating vegetables from a roadside garden is safe. Abstract statistics from formal risk assessments are of little use to them. Informing the public about research findings and regulatory plans in an understandable way could ease political pressures on EPA and the states to take actions that may not be warranted scientifically. Communication between EPA and the states should also be improved. Representatives from state environmental offices would like to take a more active role in EPA's conceptual planning. In general, EPA tells the states what, where, and when to monitor. Yet state officials know best what monitoring is already going on, what the primary concerns are in their region, and what resources are available without having to divert funds from higher-priority activities.

In general, symposium participants recognized the need for additional air toxics research and regulation. A few participants recommended keeping concern about air toxics in perspective. One demonstration showed that only a small fraction of all deaths from cancer is attributable to air toxics. However, other participants expressed concern that air toxics pose a significant health risk and deemed them a "sleeping giant." For years, EPA has focused on the criteria air pollutants--first carbon monoxide, then ozone, and most recently particulates. In response to EPA mandates, states have put substantial resources toward meeting standards for those pollutants. Meanwhile, the air toxics have been largely overlooked. Given the potentially serious health impacts of air toxics, allocating more resources to controlling these "poor cousins" of the criteria pollutants may well be warranted.

This article is based on a summary of the 1999 Urban Air Toxics Summer Symposium that was prepared by Renee J. Robins, program coordinator for MIT's Integrated Program on Urban, Regional, and Global Air Pollution. The summary is available as a pdf file at the Energy Laboratory's home page, located at <http://web.mit.edu/energylab/www/>. The symposium was held at Endicott House in Dedham, Massachusetts, on July 8-9, 1999. It was funded by the MIT Energy Laboratory, MIT Center for Environmental Initiatives, EPA Center on Airborne Organics, EPA Office of Research and Development, Northeast States for Coordinated Air Use Management, California Air Resources Board, The Health Effects Institute, Mickey Leland National Urban Air Toxics Research Center, US Department of Transportation Volpe National Transportation Systems Center, American Petroleum Institute, Chevron, British Petroleum, Amoco, Sun Oil Company, Engine Manufacturers Association, Chemical Manufacturers Association, Electric Power Research Institute, and Pennsylvania Power & Light Company.