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On particularly smoggy days, small children and people with respiratory illness are encouraged to stay indoors to avoid health risks from air pollution. While this may be good advice if the goal is to reduce exposures to, let's say, ozone, there is substantial evidence that concentrations of many airborne pollutants are often higher inside buildings and vehicles than outside. It is becoming increasingly apparent that being indoors, as, for example, in a residence, office, or automobile, can offer protection from exposure to some airborne agents, while at the same time increasing exposure to others. Concerns about the healthfulness of indoor air are driven by six major factors: First, it is now widely recognized that most people spend more than 90 percent of their time indoors. Groups potentially more susceptible to the effects of air pollution, like infants, the infirm, and the elderly, are inside virtually all the time. Because most of us spend so much time inside, indoor pollution concentrations, even if they are uniformly lower than outdoor levels, make a significant contribution to our average exposure over a day, week, season, or year. Second, modern indoor environments contain a complex array of potential sources of air pollution, including synthetic building materials, consumer products, and dust mites. Airborne emissions also occur because of the people, pets, and plants that inhabit these spaces. Efforts to lower energy costs by reducing ventilation rates have increased the likelihood that pollutants generated indoors will accumulate. Third, monitoring studies inside buildings and vehicles have consistently found that concentrations of many air pollutants tend to be higher indoors than out. Indoor air has been shown to be a complex mixture of chemical, biological, and physical agents, only a small fraction of which has been characterized adequately. This complexity is illustrated by the fact that more than 4,000 different compounds have been identified in tobacco smoke alone. Fourth, scientific reports indicate that indoor measurements are often better than outdoor measurements for classifying, estimating, and predicting human exposures to many air pollutants. This is true even for some r agents that are primarily of outdoor origin. A promising approach to more realistic exposure estimation is the development and refinement of models that combine information about pollutant concentrations in both indoor and outdoor settings with data on time-activity patterns. Fifth, among the factors driving indoor air concerns, complaints about inadequate indoor air quality and associated discomfort and illness are a burgeoning problem in our society. Reports of illness outbreaks among building occupants, particularly office workers, with no secondary spread of the illness to others outside the building with whom affected individuals come into contact, have become commonplace. EPA classifies these reports into two general categories. Building-related illness refers to episodes when symptoms of diagnosable illness are identified and can be attributed directly to airborne contaminants in the building. In contrast, sick-building syndrome is defined as situations in which building occupants experience acute health and comfort effects that appear to be linked to time spent in the building, but no specific illness or cause can be identified. The so-called chemical sensitivity syndromes, which may be caused only partially or not at all by chemicals, are a different, although potentially related matter. Broadly defined,"multiple chemical sensitivity" (MCS) is postulated to be development of responsiveness, including manifestation of often disabling symptoms, to extremely low concentrations of chemicals following sensitization. A controversial and emotional topic, the concept of MCS as a distinct entity caused by exposure to chemicals has been challenged by the medical and scientific communities, and there appears to be consensus that substantially more study is needed before MCS should be considered as a clinical diagnosis. Nevertheless, many sufferers of MCS continue to believe that their conditions are either caused or exacerbated by indoor air pollution. Sixth, exposures to many indoor air pollutants are known or suspected to occur at levels sufficient to cause illness or injury. Scientific evidence suggests that respiratory disease, allergy, mucous membrane irritation, nervous system effects, cardiovascular effects, reproductive effects, and lung cancer may be linked to exposures to indoor air pollutants. Scientists consistently rank indoor air pollution at or near the top of environmental health risks in the United States. EPA reports on risk-based priority setting, like Unfinished Business (1987), Reducing Risk (1990), and the Regional Comparative Risk Projects (1989-92), all ranked indoor air pollution as a high-priority risk to human health. Public opinion polls, however, continue to find that most Americans do not perceive the risks of indoor air pollution to be great. The specter of potential public health risks from contaminated indoor air presents decision makers with a dilemma. Is the problem serious enough to warrant intervention, and, if so, what preventive or remedial actions are most appropriate? The significance of indoor air exposures for acute and chronic health effects remains uncertain in most cases. Nevertheless, there is ample reason for concern and caution. For example, it has been estimated that exposures of nonsmokers to may cause as many as 3,000 lung cancer deaths annually in the United States, as well as contribute to a wide range of non cancer diseases, including pneumonia, bronchitis, and asthma. Findings from several studies suggest that indoor concentrations of nitrogen dioxide, carbon monoxide, and respirable particles can exceed the National Ambient (Outdoor) Air Quality Standards set by EPA to protect public health. And results from many studies show that a plethora of volatile organic chemicals and pesticides known to be toxic and/or carcinogenic can occur indoors at concentrations significantly higher than levels that create concerns in outdoor air. Designing effective and efficient indoor control strategies requires an understanding of several pertinent factors. Contaminant characteristics need to be considered, including factors like concentration, reactivity, and physical state. Emission source configurations (e.g., area or point sources) must be taken into account, and it is necessary to determine whether discharges are continuous or intermittent. It is also important to understand the dose-response relationship for the contaminant of interest so that informed decisions can be made whether individuals are to be protected from short-term exposures to peak concentrations or from long-term exposures to relatively low concentrations. And, of course, the type of indoor enclosure (e.g., residence, office, car) has ramifications for which approaches and methods are viable options. But providing and maintaining healthful indoor air quality is more than just a complex scientific and technical issue. Realization that contaminated indoor air may pose an unacceptable health hazard raises complicated policy questions about the proper role of government in safeguarding people's health inside public and private spaces. Because concerns about adverse health consequences from air pollution have focused traditionally on outdoor and occupational exposures, federal and state government programs concentrate on protecting public health from outdoor air pollutants or protecting workers’ health from dangerous air pollutants in the industrial workplace. As mentioned earlier, EPA sets and enforces National Ambient Air Quality Standards that are designed to protect the general public to within an adequate margin of safety. The Occupational Safety and Health Administration enforces consensus standards for industrial work environments, which are designed such that no employee will suffer material impairment of health or functional capacity. However, federal responsibility and authority for indoor air quality in the non workplace are less well defined. There is ample precedent for government authority and responsibility to protect public health and welfare inside buildings. It is common practice, for instance, to regulate construction and operation of public buildings to ensure that adequate provisions are made for health and safety. Government inspectors routinely enforce building codes, health regulations, safety rules, and fire ordinances. While government has an obligation to protect public health in indoor as well as outdoor environments, the justification for direct government intervention varies according to the characteristics associated with different types of indoor settings. Creation of a regulatory framework for protecting indoor environmental quality poses special policy issues. Promulgation of indoor air quality standards and other regulations must acknowledge that individuals, especially in private residences, are already making decisions about their own air quality. Development of effective and reasonable policy requires an appreciation of the scope for private action, as well as consideration of the likelihood that public intervention will foster improved private choices. This is not to suggest that rules and regulations have no part to play in safeguarding indoor air quality. This form of intervention is, however, not necessarily optimal or even desirable in certain types of indoor environments. There are, of course, many different types of indoor environments for example, occupational settings, both industrial and non industrial; non occupational settings, including residential, commercial, institutional, and public; and transportation micro environments, such as automobiles, airplanes, subways, and trains. The role of government varies according to the "publicness" of a particular space as well as the nature of air-pollution health risks, either voluntary or non voluntary. Understanding the diversity of nonindustrial indoor environments is an important step in the design and implementation of practical and cost-effective control strategies. The rationale for government regulation of outdoor air pollution is based in part on a definition of outdoor air as a "public good" and on the realization that those who suffer the effects of such pollution are neither compensated by, nor powerful in influencing, polluters. The situation is quite different for some indoor environments, especially private residences, for both the costs and benefits of pollution control are internalized with households. If occupants foul the air in their home, they are forced to breathe it. If they attempt to improve its quality by increasing ventilation or installing air-cleaning devices, they bear the costs and enjoy the benefits. For some contaminants, such as tobacco smoke, odorants, and water vapor, benefits are readily recognizable through improvements in perceptible air quality and reduction of corrosion, soiling, and molds. The closed-loop, cost-benefit cycle within residences suggests that individual decisions are important determinants of indoor air quality. However, unlike residential energy consumption, where monthly bills from the local utility company provide periodic feedback to consumers, indoor contaminants may be below irritation or odor thresholds. Thus, although individuals are certainly making decisions about their own air quality, it is not clear that these are "informed" decisions. Government actions aimed at improving personal decisions about indoor air quality may be preferable to rules and regulations (e.g., simple warning devices, product labeling, or information programs). It has been suggested that the Clean Air Act be amended to give EPA authority to control indoor air pollution in much the same way that outdoor air pollution is currently controlled. However, setting and enforcing strict indoor air quality standards, similar to existing National Ambient Air Quality Standards, would be impractical because of the prohibitive monitoring costs and the difficulty of enforcement within approximately 82 million residences in the United States. Perhaps the most serious impediment to implementing a regulatory approach is public antipathy towards this form of intervention inside the home. Restriction of indoor pollution sources, certification of "safe" indoor concentrations, product emission standards, disclosure of potential sources upon transfer of ownership, and specification of minimum ventilation requirements are examples of government actions that are likely to be less costly and more effective than indoor air quality standards. Not all buildings are residences, and not all residences are owner-occupied. The rationale for direct government intervention aimed at improving indoor air quality is much stronger in public, as opposed to private buildings. Air quality in large public buildings, for instance, displays many characteristics of a public good. A person sensitive to tobacco smoke would not rationally pay the costs of cleaning the air in a large convention hall. The costs would greatly exceed any personal benefits an individual might derive from smoke-free air, and those who did not contribute could not be excluded from enjoying the benefits. In this situation, the rationale for regulation is similar to that for outdoor air pollution. There is also substantial justification for regulatory intervention in private and public buildings where occupants do not have control over their own environment, for example, modern high-rise office buildings. Typically, building managers are responsible for operation and maintenance of heating, ventilation, and air-conditioning (HVAC) systems. Occupants of the building, including both employers and employees, often have little or no direct control of temperature, fresh air input, and ventilation rate. Because HVAC systems are normally operated to minimize energy costs, the health and comfort of tenants rarely become an issue unless a significant number of complaints are reported. Because health risks in this situation tend to be non voluntary, government may have a responsibility to safeguard public health by defining what constitutes acceptable indoor air and taking steps to ensure that those criteria are met. Examples of government actions that might be warranted include specification of minimum ventilation rates necessary to achieve healthful indoor air quality, establishment of emission limitations for building materials, and development of indoor air quality guidelines or standards for important contaminants. As a practical matter, however, development of a comprehensive federal approach to address problems of indoor air pollution awaits resolution of important public policy and public health questions about the proper role of government in safeguarding the air quality inside public and private spaces. Key Questions About Indoor Air Pollution Answers to the following key questions are critical to improving our understanding of the relative risks associated with indoor air pollution; whether these risks are unacceptable; and what to do about those that are. Problem Definition: Is there an existing or potential indoor air problem that may have public health significance?
Risk Assessment: If indoor air poses serious risks to human health, what are the likelihood and magnitude of those risks?
Risk Management: If the associated human health risks are deemed unacceptable, what are the most effective approaches to preventing/reducing these risks?
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