Changes in indoor pollutants since the 1950s
Introduction
The chemicals found indoors are constant in neither kind nor concentration. Changes occur day-to-day, month-to-month, year-to-year and decade-to-decade. Chemicals that building occupants are exposed to today are substantially different from those that occupants experienced 50 years ago. Knowledge of such differences can aid in unraveling the effects that pollutants have on multiple aspects of human health.
This review presents general trends in the concentrations of indoor pollutants since the 1950s. It focuses on the United States, but the trends discussed have also been observed in other industrialized countries. Some of the restrictions or bans on certain chemicals may have occurred earlier or later in different parts of the world, but, to a large extent, the same chemicals have eventually been regulated. Many of the manufacturers of the materials, furnishings and products used indoors are international. Indeed, globalization has increased the extent to which indoor environments in the United States, Europe, Asia and other parts of the world have come to resemble one another.
Initially, because of concerns with outdoor pollution and the understanding that outdoor pollution impacted indoor environments, the indoor pollutants that received the greatest attention were noxious substances that originated outdoors, especially sulfur dioxide, nitrogen oxides, ozone and airborne particles (Biersteker et al., 1965, Andersen, 1972 and references therein; Yocom et al., 1971, Sabersky et al., 1973, Thompson et al., 1973, Shair and Heitner, 1974). Subsequently attention turned to pollutants that were of particular concern indoors and readily measured; these included formaldehyde, radon, asbestos, tobacco smoke and nonpolar volatile organic compounds (National Research Council, 1981 and references therein). Over time, pesticides (Lewis, 2001 and references therein) and other semivolatile organic compounds (Weschler, 1980, Weschler, 1984, Lioy et al., 1985) were measured indoors. As better analytical instruments were developed and instrument sensitivities improved, the number and types of compounds measured indoors increased. This was particularly true for organic compounds measured by capillary gas chromatographs interfaced to mass spectrometers. Presently researchers have begun to apply sophisticated techniques such as Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and Atmospheric Sampling Townsend Discharge Ionization Mass Spectrometry (ASTDI-MS) to measure species anticipated to be present in certain indoor settings (e.g., Wisthaler et al., 2005, Nøjgaard et al., 2007). Nonetheless, there remain compounds whose levels have not been directly measured, and whose presence indoors is only inferred (e.g., hydroxyl, nitrate, hydroperoxy and methylperoxy radicals).
How can we discuss indoor pollutant trends over a time-span that includes decades (i.e., prior to the 1970s) with few, if any, measurements of chemicals in indoor air? One way is to examine production figures for different chemicals over the time period of interest. This is a particularly valuable approach for chemicals that have primarily indoor uses such as certain plasticizers or flame-retardants. We can also look at the building materials that were common at different points in time. Given the composition of these materials, we can infer the major chemicals that they emitted. A similar approach applies to wall assemblies, floor coverings, architectural coatings, furnishings, cleaning agents and other products used indoors. However, the reader is cautioned that many of the inferences contained in this paper are “best judgments” about likely changes and do not have the certainty of findings based on direct empirical evidence.
The concentration of an indoor pollutant depends not only on its indoor emission rate, but also on the rate at which it is being transported from outdoors to indoors, and the rates at which it is scavenged by indoor surfaces, consumed by indoor chemistry and removed by ventilation or filtration. Changes in these source and sink terms are also examined in the present review.
Table 1 presents an admittedly subjective list of major events, actions and regulations that have affected the concentrations of pollutants in U.S. homes, offices and schools. Some of the entries refer to changes in the way that buildings were operated (e.g., air conditioning) or constructed (e.g., increased use of composite-wood products). Some refer to changes in the way that products were formulated (reduction of lead and mercury in paint, increased presence of synthetic fibers in carpets). Some refer to regulations that limited the use of certain products (pesticides, asbestos, chlorofluorocarbons). Some are deemed significant because they altered people's thinking about environmental pollution in general (publication of “Silent Spring”) or indoor pollution specifically (publication of National Research Council report “Indoor Pollutants”). Some of the events listed were international in scope. Others are specific to the U.S., although analogous events often occurred in other countries. While a number of these will be called out in the course of this review, many are self-explanatory, and no further discussion will be provided. Regardless, stepping through the entries in Table 1 provides a sense of how and why an indoor environment in 2008 is so different from its counterpart in the early 1950s.
Section snippets
Building materials
Numerous building materials emit chemicals into indoor air (Levin, 1989). This sub-section briefly discusses three that have come to dominate their respective categories, but were largely absent from buildings prior to the 1950s – composite-wood, PVC pipes and PVC wire/cable insulation.
Composite-wood. Following World War II, plywood began to replace solid wood in home construction, and, in the period from 1954 to 1975, U.S. plywood production rose from 4 billion to 16 billion ft2 year−1 (0.4–1.5
Oxidation reactions
Chemical reactions among indoor pollutants alter the mix and concentrations of indoor pollutants (Weschler and Shields, 1997, Weschler, 2004). Indoor oxidation reactions have received the greatest attention to date. Oxidation products include free radicals, secondary ozonides, epoxides, aldehydes, ketones, acids, diacids, dicarbonyls and other oxygenated species (Weschler, 2000, Weschler, 2006). Some of the products have low vapor pressures and contribute to the growth of secondary organic
Smoking
In 1964, the first U.S. Surgeon General's report on “Smoking and Health” was issued. In 1973, the state of Arizona restricted smoking in public places. In 1994, the state of California restricted smoking in workplaces. In 2003, New York City amended its smoking restrictions to include all restaurants and bars. Today many U.S. state and local governments, as well as many businesses, have bans on smoking in various indoor environments. Furthermore, as illustrated in Fig. 2, the percentage of the
Tighter buildings
Steps to tighten building envelopes were included in energy conservation measures implemented following the Arab Oil embargo of 1973. Residential buildings constructed in the past two decades tend to be tighter and have lower air exchange rates than buildings constructed in the 1950s, 1960s and early 1970s (Weisel et al., 2005). Although conventional wisdom holds that the same trend applies to non-residential buildings, in a study of data from 139 commercial buildings, no correlation was seen
Trends in indoor pollutants
Table 2 lists selected indoor pollutants and, for each pollutant, an up or down arrow to broadly summarize the trend in its indoor concentration since the 1950s. Most of these trends have been inferred; there are only a small number of pollutants for which indoor measurements exist over an extended period of time. The paragraphs in this section provide supporting information for the “best judgments” presented in the table. Compounds in Table 2 preceded by an asterisk are those for which
Conclusions
The health risks from indoor pollutants in 2008 differ from those in the 1950s. Indoor exposures to a number of “known” carcinogens (e.g., benzene, formaldehyde, asbestos, environmental tobacco smoke and radon) and “reasonably anticipated” carcinogens (e.g., chloroform, trichloroethylene, carbon tetrachloride and naphthalene) have decreased. Indoor exposures to other recognized toxicants such as carbon monoxide, sulfur dioxide, nitrogen dioxides, lead and mercury have also declined. Conversely,
Acknowledgements
I thank Hal Levin for multiple discussions and extensive suggestions regarding the material presented in this review. William W Nazaroff, Tunga Salthammer, Cliff Weisel, Armin Wisthaler and Louise B. Weschler provided further valuable input.
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