Abstract
Improving air quality by decreasing air pollutant concentration levels requires promulgating regulations that protect public health, ecosystems, the agriculture, buildings, and atmospheric visibility. Then, public policies must be implemented to design, apply, and evaluate emission control strategies to meet the regulatory standards. In this chapter, the general approach for the development of regulations to protect public health via ambient air quality standards is described first. These regulations and the implementation of the associated public policies differ among countries. Those used in the United States and in France are presented here comparatively to illustrate slightly different approaches. Finally, approaches used at the national and international levels to regulate atmospheric deposition and global atmospheric issues (i.e., destruction of the stratospheric ozone layer and climate change) are presented.
Improving air quality by decreasing air pollutant concentration levels requires promulgating regulations that protect public health, ecosystems, the agriculture, buildings, and atmospheric visibility. Then, public policies must be implemented to design, apply, and evaluate emission control strategies to meet the regulatory standards. In this chapter, the general approach for the development of regulations to protect public health via ambient air quality standards is described first. These regulations and the implementation of the associated public policies differ among countries. Those used in the United States and in France are presented here comparatively to illustrate slightly different approaches. Finally, approaches used at the national and international levels to regulate atmospheric deposition and global atmospheric issues (i.e., destruction of the stratospheric ozone layer and climate change) are presented.
15.1 Regulations for Air Pollutant Concentrations
A regulation of ambient air pollution includes six components:
The regulated air pollutant, called the indicator species
The exposure duration
The regulatory value
The statistical form of the regulation
The location of the monitoring stations
The measurement method
15.1.1 Indicator Species
Regulations are set either for a specific air pollutant (for example, lead or benzene) or for a group of air pollutants. The latter include, for example, photochemical oxidants, nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter (PM). In the case of a group of pollutants, a representative pollutant must be selected. For photochemical oxidants, ozone (O3) is used because it is the chemical species of that group that is present in the highest concentrations. For NOx, nitrogen dioxide (NO2) is used because it shows well-documented adverse effects on the human respiratory system. For SOx, sulfur dioxide (SO2) is used because it also shows well-documented adverse effects on the human respiratory system. Sulfur trioxide (SO3) has a very short atmospheric lifetime, because it is rapidly hydrolyzed to sulfuric acid (H2SO4). H2SO4 is present in the atmosphere in the particulate phase because of its very low saturation vapor pressure. Particles lead to adverse respiratory and cardio-vascular effects; therefore, particles are regulated separately. Thus, it is appropriate to select SO2 as the indicator species for SOx. In the case of particles, the regulations have targeted their size rather than their chemical composition up to now. The reason is that there is a large body of health studies that associate adverse health effects with inhalable particles (PM10) and fine particles (PM2.5), whereas there is currently insufficient epidemiological evidence to identify the adverse health effects of individual constituents of PM (one exception could be diesel particles).
15.1.2 Exposure Duration
Health effects may result from acute exposure (ranging from a few minutes to a few hours) and/or chronic exposure (ranging from a few months to a few years). Therefore, the regulation must correspond to the exposure duration that is considered representative of the health effects. For some air pollutants, regulations may be appropriate for both short (acute exposure) and long (chronic exposure) durations. Table 12.2 summarizes the exposure durations relevant to health effects due to air pollutants regulated in North America and in Europe. In some cases, a pollutant may be regulated for an exposure duration in the absence of conclusive evidence of adverse health effects (see Tables 12.2, 15.1, and 15.2). Such cases (e.g., the annual standard for NO2 in the United States and in Europe and the annual standard for PM10 in Europe) generally result from historical reasons (earlier health data led to the regulation, which is then kept by default). The regulation that corresponds to the exposure duration with the best evidence of adverse health effects is typically the most constraining. On the other hand, it may happen that a regulation is not set for both acute and chronic exposures in spite of epidemiological evidence for both. This is the case, for example, for PM2.5 in Europe, where only a long-term standard is used (see Tables 12.2 and 15.2). This choice may, however, be appropriate because the long-term exposure is considered to correspond to the most important adverse health effects. In the United States where both short- and long-term exposures to PM2.5 are regulated, the long-term regulation (annual concentration averaged over three years) is the most constraining and it is rare that a region would be in attainment of the annual standard, but would not be in attainment of the short-term (daily) standard. It is also the case for ozone (O3), which is only regulated for short-term exposure (8-hour average concentration), although there is some evidence of likely adverse health effects for chronic exposure to ozone (see Table 12.2). It is considered that an 8-hour average regulatory value also protects against chronic exposure, based on air quality data analyses relating ozone concentrations averaged over short and long periods.
Table 15.1. National ambient air quality standards (NAAQS) to protect public health in the United States (Clean Air Act and regulations of the U.S. Environmental Protection Agency).
| Pollutant | Concentrationa | Sampling duration | Statistical formb (number of authorized exceedances per year) |
|---|---|---|---|
| Pb | 0.15 μg m−3 | 3 moc | (0)d |
| CO | 40 mg m−3, 35 ppm | 1 h | 99.99th percentile (1)e |
| 10 mg m−3, 9 ppm | 8 hc | 99.99th percentile (1)e | |
| SO2 | 197 μg m−3 75 ppb | 1 h | 99th percentile (3)f |
| NO2 | 189 μg m−3, 100 ppb | 1 h | 98th percentile (7)g |
| 100 μg m−3, 53 ppb | 1 year | (0)d | |
| O3 | 137 μg m−3, 70 ppb | 8 hc | 99th percentile (3)h |
| PM10 | 150 μg m−3 | 24 h | 99.7th percentile (1)i |
| PM2.5 | 35 μg m−3 | 24 h | 98th percentile (7)j |
| 12 μg m−3 | 1 year | (0)d, k |
(a) regulatory concentrations are expressed in μg m−3 for Pb and PM and in ppb or ppm for gaseous pollutants; conversion from ppb (or ppm) to μg m−3 (or mg m−3) is at 25 °C and 1 atm. Concentrations are measured at background monitoring stations except for the concentrations of NO2 and PM2.5, which must be measured near sources (i.e., mostly near roadways), and for the concentrations of SO2, which must be both measured and modeled near sources.
(b) the statistical form is provided here both as a percentile and a number of exceedances per year; see footnotes for the exact definition.
(c) moving average.
(d) not to be exceeded.
(e) not to be exceeded more than once per year.
(f) 99th percentile of daily maximum 1-hour average concentrations averaged over 3 years; 24-hour average and annual standards were eliminated in 2010 (except for non-attainment areas).
(g) 98th percentile of daily maximum 1-hour average concentrations averaged over 3 years.
(h) annual fourth-highest daily maximum 8-hour average concentrations averaged over 3 years.
(i) not to be exceeded more than once per year averaged over 3 years.
(j) 98th percentile averaged over 3 years.
(k) annual mean averaged over 3 years not to be exceeded.
Table 15.2. Ambient air quality standards to protect public health in Europe (European Directive).
| Pollutant | Concentrationa | Sampling duration | Statistical formb (number of authorized exceedances) |
|---|---|---|---|
| Pb | 0.5 μg m−3 | 1 year | (0)c |
| CO | 10 mg m−3, 9 ppm | 8 hd | (0)c |
| SO2 | 350 μg m−3, 133 ppb | 1 h | 99.7th percentile (24)e |
| 125 μg m−3, 47 ppb | 24 h | 99.2th percentile (3)f | |
| NO2 | 200 μg m−3, 106 ppb | 1 h | 99.8th percentile (18)g |
| 40 μg m−3, 21 ppb | 1 year | (0)c | |
| O3h | 120 μg m−3, 61 ppb | 8 hd | 93th percentile (25)i |
| PM10 | 50 μg m−3 | 24 h | 90.4th percentile (35)j |
| 40 μg m−3 | 1 year | (0)c | |
| PM2.5k | 25 μg m−3 | 1 year | (0)c |
| C6H6 | 5 μg m−3, 1.5 ppb | 1 year | (0)c |
(a) all regulatory concentrations are expressed in μg m−3 (or mg m−3); conversion of gaseous pollutant concentrations from μg m−3 (or mg m−3) to ppb (or ppm) is at 25 °C and 1 atm.
(b) the statistical form is provided here both as a percentile and a number of exceedances per year; see footnotes for the exact definition.
(c) not to be exceeded.
(d) daily maximum 8-hour (moving) average concentrations.
(e) not to be exceeded more than 24 times per year.
(f) not to be exceeded more than 3 times per year.
(g) not to be exceeded more than 18 times per year.
(h) target value.
(i) not to be exceeded more than 25 days per year averaged over three years.
(j) not to be exceeded more than 35 times per year.
(k) limit value of 25 μg m−3; target value of 20 μg m−3 (three-year average at urban background monitoring stations) in 2020.
15.1.3 Regulatory Value
The regulatory value (the value of the national ambient air quality standard in the U.S., the limit or target value in Europe) is the best-known aspect of the air pollutant regulation. However, it cannot be dissociated from the exposure (or sampling) duration, the statistical form of the regulation (see Section 15.1.4), or the location of the monitoring stations (see Section 15.1.5). The regulatory value is determined from toxicological and/or epidemiological studies. For standards corresponding to durations of 24 h or more, epidemiological studies are typically used, since human toxicological studies are not conducted for exposure durations exceeding a few hours. For shorter exposure durations, toxicological studies may be favored. However, epidemiological studies add some useful information concerning sensitive populations, which may not be available from toxicological studies. In addition, experimental air quality data may be used to extrapolate ambient pollutant concentrations temporally or spatially, in order to relate results available from toxicological or epidemiological studies with some characteristics of the regulation (e.g., sampling duration or location; see examples later in this section). Studies used to set up regulatory values in the United States are summarized in Table 15.3.
Table 15.3. Data used to define the regulatory values of the national ambient air quality standards in the United States. Source: Clean Air Act and regulations of the U.S. Environmental Protection Agency, Federal Register.
| Pollutant and sampling duration | Toxicological studies | Epidemiological studies | Ambient concentrations |
|---|---|---|---|
| Pb, 3 months | ✓ | ✓ | |
| CO, 1 h | ✓ | ||
| CO, 8 h | ✓ | ||
| SO2, 1 h | ✓ | ✓ | |
| NO2, 1 h | ✓ | ✓ | ✓ |
| NO2, 1 year | ✓ | ||
| O3, 8 h | ✓ | ✓ | |
| PM10, 24 h | ✓ | ||
| PM2.5, 24 h | ✓ | ||
| PM2.5, 1 year | ✓ |
Toxicological studies (clinical studies conducted under controlled exposure) have been used for O3 (concentration averaged over eight hours) and CO (concentrations averaged over one and eight hours). On the other hand, epidemiological studies have been used for the NO2 and PM2.5 annual values. Epidemiological studies were used in combination with a model representing the relationships between atmospheric concentrations, exposure, blood concentrations, and adverse health effects for Pb (3-month moving-average concentration) in order to better account for various possible exposure pathways. Epidemiological studies have also been used for the 24-hour average values of PM2.5 and PM10.
In some cases, toxicological and epidemiological studies are used in combination. In the case of the hourly NO2 value, toxicological studies provided the low value of the range of concentrations corresponding to adverse health symptoms for individuals with moderate asthma. Epidemiological studies were then used to better account for more sensitive individuals (e.g., severe asthma). The regulatory value was to be set for near-source situations. Since the epidemiological studies were mostly based on existing air quality monitoring networks, which only included urban background locations, the analysis was complemented with an analysis of air quality data to extrapolate the urban background data to near-roadway locations. In the case of the hourly SO2 regulatory value, toxicological studies have also provided the low value of the range of concentrations leading to adverse health effects for individuals with moderate asthma exposed for short periods (5 to 10 minutes). Since the regulatory value was set for a one-hour average concentration to minimize temporal fluctuations in the measured concentrations, ambient concentration data were used to relate the 5-minute average toxicological data and the 1-hour ambient concentrations. In the case of O3, epidemiological studies have been used only to complement toxicological studies to confirm that significant adverse health effects do not appear below the low value of the range of concentrations proposed for the standard.
In summary, the U.S. regulations are based on epidemiological studies for concentrations averaged over periods ranging from 24 hours to one year. They are mostly based on toxicological studies for concentrations averaged over periods ranging from one to eight hours. However, those latter regulatory values are in some cases also based on epidemiological studies that provide useful information concerning sensitive individuals, who cannot participate in toxicological studies. In addition, air quality data have been used to extrapolate the results of health studies spatially (in the case of NO2) or temporally (in the case of SO2).