Developing a Framework for Effective Air Quality Management

3.3 Setting Air Quality Goals for a Region

3.3.3 Defining the Format of Air Quality Standards
Obviously, each type of air pollution problem may call for a different form of standard. For many pollutants a maximum allowable level of pollutant in the air measured over a specific time period is set as the ‘standard’. This is the approach used by the United Nations and most countries for common air pollutants. The level of pollutant in the air is usually described as the amount of pollutant per unit air, or the pollutant concentration in the air. However, there are many cases where concentration limits are not the best approach for setting some standards, as will be discussed later.

When concentration limits appear to be the most logical approach, it is also necessary to define an associated averaging time. Humans, animals, and plants can often stand lower doses of air pollution over short timeframes with little or no damage. Thus, with each standard must come a time period over which the standard is applicable. One hour, eight hours, twenty-four hours, and one-year have all been used as an appropriate averaging time for different pollutants. Sometimes, it is necessary to define two or more standards for a single pollutant. For example in the United States, there is a standard for ozone of 85 ppb over 8 hours, and 120 ppb over 1 hour. In this situation, both standards are necessary because ozone is harmful at the higher level of 120 ppb in just a one hour period; yet 85 ppb can cause harm over an 8 hour period even if the 120 ppb level is never reached. Once standards and timelines are decided, then it is necessary to determine if compliance with the standard on every hour or day of the year is necessary or if some number of violations of the standard is to be allowed in a given year or over multiple years, to account for the occasional odd weather condition or emissions event. Some standards allow no excedances, while some standards allow one or more excedance per year or in a three-year period.

A concentration limit carries with it the implication that there is a safe level for a pollutant (i.e. air quality with levels of pollutants below the standard). This may not be the case. Combinations of pollutants can be a problem even at very low levels, and research indicates that there may be some pollutants for which there is no safe level. Thus, a concentration limit can be deceptive. Some researchers have argued that fine particulate matter has no safe level. It appears in some studies that some sensitive persons can be seriously injured or killed at any measurable level of fine particulate matter. Similarly, some researchers have argued that there is no safe level of a pollutant that causes cancer. Instead, as the levels of a cancer causing pollutant increases, the probability of an exposed person getting cancer may increase, but there is a small but finite probability of cancer forming even at the lowest measurable level. This has led to attempts to develop standards for some pollutants in terms of the probability of causing a negative health effect such as requiring levels of a carcinogen such that the risk of a person getting cancer over 70 years of exposure is less than one in one million.

Visibility and odor standards are more subjective and involve more than one pollutant. Thus, they are not amenable to concentration standards. In the case of visibility, visual range (the distance that a large object can still be perceived) or some form of light scattering measurement is a common way to establish a standard. In the case of odors, criteria based on panels of judges or dilution of a sample until it cannot be perceived have been used to set standards.

The processes associated with global climate change are associated with a number of pollutants and are not understood well enough to establish clear concentration limits for the various contributors. Thus, carbon dioxide (CO2) is normally used as a proxy since it is the most common global warming pollutant. Climate change pollutants such as natural gas and N2O are then usually equated to the amounts of CO2 that produces an equivalent climate (heat absorbing) impact. Then goals are established in terms of CO2 equivalent emission rates compared to some fixed point in time. For example, 1990 was considered as an important benchmark for the Kyoto discussions. Some climate change programs simply set overall emission reduction goals rather than CO2 concentration goals.

Concentration standards are typically set as volume or as weight measurements. For example, 250 ppmv (parts per million by volume) is a form of volume measurement. This is the same as saying that there are 250 milliliters of the pollutant found in a million milliliters of air. Another way of thinking about this is that it is the number of molecules of the pollutant found in a million molecules of collected air. 100 micrograms/cubic meter is a weight measurement and means that the pollutant in one cubic meter of air weighs 100 micrograms. These measurement units and processes used to set the standards are discussed in more detail in Chapter 2.