Outdoor Air Pollution - America's Children and the Environment: A First View of Available Measures

Common Air Pollutants

Air pollution contributes to a wide variety of health effects, though most of the evidence for health impacts is from studies on adults. The most common air pollutants - ground-level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide - are regulated by EPA and the individual states. These pollutants often are called criteria pollutants.

Several of these pollutants, including ozone and particulate matter, have been associated with increases in respiratory-related diseases in children, including reduction of lung function, increases in respiratory symptoms, and increased severity or frequency of asthma attacks. ^1-9 Lead damages the central nervous system in children. Higher concentrations of particulate matter increase mortality in the general population. ^10-11

EPA sets National Ambient Air Quality Standards for each of these pollutants to protect people from adverse health effects. The standards specify how much of each pollutant is allowed in the air. Some of the standards are designed to protect the public from adverse health effects that can occur after being exposed for a short time, such as one hour or one day. Other standards are designed to protect people from health effects that can occur after being exposed for a much longer time, such as a year. For example, current standards for carbon monoxide are for periods of one hour and eight hours, while the current standard for nitrogen dioxide is for one year. The standards and the varying time periods for which they apply are shown in Appendix B as Table 1.

Air Quality Exceedances During a Year

State agencies that monitor air quality report their results to EPA, which then reports instances in which the measured concentration of a pollutant exceeds the standard for that pollutant. A description of the methods used to determine whether an exceedance has occurred is available in Appendix B.

For this measure, we used EPA's results showing when air quality standards were exceeded in counties in the United States. We calculated the percentage of children living in areas with reported exceedances for the six criteria pollutants. This measure shows the percentage of children that may be exposed to poor air quality at some point during a year.

This measure does not differentiate between areas in which standards are exceeded frequently or by a large margin and areas in which standards are exceeded only rarely or by a small margin. Also, because the nature of health effects varies significantly and the averaging times associated with different standards vary widely, exceedances for different standards are not comparable. For example, the ozone standard considers measured levels of ozone within a one-hour period and health effects such as lung function decrements, respiratory symptoms, and hospital admissions. In contrast, the averaging time for the lead standard is three months and is based on health effects such as IQ decrements and hypertension.

Measure
E1: Percentage of children living in counties in which air quality standards
were exceeded

From 1990 through 1998, approximately 25 percent of children lived in a county in which at least one air quality standard was exceeded during the year.

The highest number of exceedances was for ozone. In 1990, approximately 23 percent of children lived in counties in which the ozone standard was exceeded on at least one day. In 1998, approximately 21 percent of children lived in such counties.

In 1990, approximately 10 percent of children lived in counties in which the carbon monoxide standard was exceeded. In 1998, approximately 4 percent of children lived in such counties.

From 1990 to 1998, the percentage of children living in counties that exceeded the daily standard for particulate matter fluctuated, but was as high as 10 percent in 1992 and 1995.

On average, 2 percent of children lived in counties that exceeded the standard for lead. The main sources of ambient concentrations of lead are metals processors, such as smelters, and battery manufacturers.

No exceedances of the nitrogen dioxide standard have occurred since 1991. However, the nitrogen dioxide standard is based on measurements over a full year and therefore is not comparable to the other standards included here. Also, few exceedances of the sulfur dioxide standard have occurred since 1993. Consequently, these two pollutants are not included on the graph.

Daily Air Quality

EPA provides an Air Quality Index (AQI) that offers useful information about air quality. The purpose of the AQI is to help individuals understand what local air quality means to their health. The AQI is like a yardstick: the higher the AQI value, the greater the level of air pollution and the greater the danger to health.

The AQI is based on measurements of up to five of the six air quality criteria pollutants (carbon monoxide, ozone, nitrogen dioxide, sulfur dioxide, and particulate matter). The AQI is a measure of air quality for each day. An AQI value of 100 for a given criteria pollutant generally corresponds to the national ambient air quality standard (NAAQS) for that pollutant and the level EPA has set to protect public health for that pollutant on a single day.

EPA has divided the AQI scale into categories. Air quality is considered "good" if the AQI is between 0 and 50. At this level, air quality is satisfactory and air pollution poses little or no risk over the short term. Air quality is considered "moderate" if the AQI is between 51 and 100. Air quality at this level is acceptable, but some pollutants may present a moderate health concern for a very small number of individuals. Moreover, such a level may pose health risks if maintained over many days. Air quality is considered "unhealthy for sensitive groups" if the AQI is between 101 and 150. Members of sensitive groups such as children may experience health effects, but the general public is not likely to be affected. Air quality is considered "unhealthy" if the AQI is between 151 and 200. The general population may begin to experience health effects, and members of sensitive groups may experience more serious health effects.

Measure E2 on the following page uses the reported AQI for the counties of the United States. This measure was developed by reviewing the air quality designation for each day for each county. The daily designations were weighted by the number of children living in each county. The resulting measure may be considered to be reported in "child-days," where the designation for each day for each child in a county is counted toward the total. This measure reflects the number of days that children live in a county with air quality in each category.

The advantage of this approach, compared with that used in measure E1 on the previous page, is that it provides a sense of the intensity of pollution over the course of a year. This method provides data on the air quality category for each day, rather than simply reporting whether a county ever exceeds standards for these pollutants.

The limitation of this method is that the AQI is based on the single pollutant with the highest value for each day; it does not reflect any combined effect of multiple pollutants. It reflects short-term, daily pollution burdens and is not well suited for reporting concentrations of lead and nitrogen dioxide because these pollutants do not have one-day standards. This approach is influenced by the frequency of measurements. Because the AQI is reported daily, pollutants that are measured daily-such as ozone-will appear to have more effect than those that are measured less frequently, such as particulate matter.

Measure
E2: Percentage of children's days with good, moderate, or unhealthy air
quality

The percentage of children's days that were designated as having "unhealthy" air quality decreased between 1990 and 1998, dropping from 4 percent in 1990 to less than 2 percent in 1998. The percentage of children's days that were designated as having "moderate" air quality remained about the same between 1990 and 1998, at about 27 percent.

The coverage of monitoring for this measure was largely unchanged between 1990 and 1998. Approximately 10 percent of children's days of exposure to air pollutants were not monitored at all. Even on days that were monitored, in many cases only one or a few pollutants were monitored. Areas that do not have monitors may be expected to have good air quality, but we do not have monitoring data to verify this.

Hazardous Air Pollutants

Hazardous air pollutants, also known as air toxics, have been associated with a number of adverse human health effects, including cancers, asthma and other respiratory ailments, and neurological problems such as learning disabilities and hyperactivity.^13-17 Examples of the 188 hazardous air pollutants listed in the Clean Air Act include benzene, trichloroethylene, mercury, chromium, and dioxins. Ambient concentrations result from emissions by local or regional sources such as chemical manufacturing plants, refineries, waste incinerators, electricity generating plants, dry cleaners, cars, trucks, and buses. For some hazardous air pollutants, ambient concentrations also result from emissions that occurred in past years or from natural sources.

Unlike the criteria air pollutants, there are no national air quality standards for hazardous air pollutants that can be used to construct a health-based measure. Instead, we have compared ambient concentrations of hazardous air pollutants with health benchmark concentrations derived from scientific assessments conducted by EPA and other environmental agencies.^{13; 18-20}

Hazardous Air Pollutants and Health Benchmarks

For this analysis we used four different health benchmark concentrations. Three benchmarks reflect potential cancer risks, at levels of one-in-a-million risk, 1-in-100,000 risk, and 1-in-10,000 risk. If a particular hazardous air pollutant is present in ambient air at a one-in-a-million benchmark concentration, for example, one additional case of cancer would be expected in a population of one million people exposed for a lifetime. The fourth benchmark concentration corresponds to the level at which a hazardous air pollutant may be associated with human health effects other than cancer.

The four benchmarks generally reflect expected effects in adults, rather than potential risks to children or risks in adulthood stemming from childhood exposure. Benchmarks are not available to reflect these concerns.

The estimates of ambient concentrations of air toxics for the year 1990 were computer-generated. The computer model provided estimates for every county in the continental United States. The computer estimates are consistent with the limited set of actual measurements of ambient air toxics concentrations available for 1990.

This measure only considers exposures to air toxics that occur by inhalation. An important additional pathway of exposure to many air toxics is through deposition of those pollutants to land and water, and subsequent accumulation in the food chain. For hazardous air pollutants that are persistent in the environment, exposures through food consumption typically are greater than inhalation exposures. Hazardous air pollutants for which these food chain exposures are important include mercury and other hazardous air pollutants that can damage a child's nervous system.^21-23

Measure
E3: Percentage of children living in counties where at least one hazardous air
pollutant concentration was greater than a health benchmark in 1990

In 1990, all children lived in counties in which the one-in-a million and 1-in-100,000 cancer risk benchmarks were exceeded by at least one hazardous air pollutant. Six percent of children lived in counties in which at least one hazardous air pollutant exceeded the 1-in-10,000 benchmark.

Approximately 95 percent of children lived in counties in which the benchmark for health effects other than cancer was exceeded by at least one hazardous air pollutant.

Actual exposures may differ from ambient concentrations. Indoor concentrations of hazardous air pollutants from outdoor sources may be slightly lower than ambient concentrations, though they can be significantly higher if any indoor sources are present. Levels of some hazardous air pollutants may be substantially higher inside cars and school buses, and those higher levels would increase the risks.

In the upcoming year, as part of its National Air Toxics Assessment (NATA) activities, EPA will finalize a national-scale assessment of hazardous air pollutant risks for the year 1996 (see http://www.epa.gov/ttn/atw/nata/ for a complete description). In the future, measures of hazardous air pollutant risks to children will be developed using information from NATA, which will be updated every three years.

Healthy People 2010:

Objective 8-01 of the Healthy People 2010 initiative aims to reduce the number of people exposed to air that fails to meet EPA's health-based standards for criteria air pollutants. See Appendix C for more information.
Objective 8-04 of the Healthy People 2010 initiative focuses on reducing emissions of hazardous air pollutants. See Appendix C for more information.