Indoor Air Quality

Indoor Air Quality: A Serious Health Issue

Introduction

Over the past two decades, outdoor air quality in the United States has improved, while indoor air quality (IAQ) has declined. Indoor air pollution is recognized as one of the top five environmental risks to public health in the United States. Americans spend about 90 percent of their time indoors where concentrations of pollutants are often much higher than those outside. Indoor pollution is associated with many health impacts, including lung cancer and various respiratory health problems. The Asthma and Allergy Foundation of America reports that the number of children with asthma has doubled in the last 15 years.

The origins of IAQ problems date back to the end of World War II when there was great demand for inexpensive housing. Homes began to be built on smaller lots with garages attached to the home, allowing automobile exhaust and evaporative emissions to enter the living space. Home construction costs were reduced by using chipped and laminated wood products and other materials bound by organic resins that emit volatile pollutants. Carpet and other floor coverings requiring adhesives provided more gaseous pollutants, as did cleaning products, personal care items, air fresheners, and pesticides.

Despite the proliferation of these indoor pollutants, IAQ problems were uncommon before the 1970s. Houses, up to this time, were relatively "leaky," and high infiltration rates served to dilute indoor pollutants. The increase in IAQ problems that began in the 1970s can be attributed to changes in home and building design in the years immediately following the oil embargo of 1973, when the public became more concerned with the cost of energy. As houses began to be built more "airtight," IAQ problems have been exacerbated by a combination of more pollutant sources and less dilution of indoor air.

Indoor Air Pollutants and Their Sources

Levels of certain pollutants in indoor air are often much higher than in outside air. The U.S. EPA reports that most Americans will experience their greatest exposure to toxic chemicals indoors. Primary sources of many of these pollutants are consumer products and building materials (Figure 1). Levels of many potentially toxic compounds typically are two to five times greater indoors than outdoors and sometimes as much as hundreds of times greater.

Radon—Radon is a colorless, odorless, and tasteless radioactive gas that cannot be detected without the use of sensitive test equipment. Radon is produced by the breakdown of naturally-occurring uranium in soil, rock, and water. Some radon passes to surrounding air and water and may enter homes through cracks in foundations, openings around sump pumps and drains, crawl spaces, and cracks in basement floors and walls. The Surgeon General and the National Academy of Sciences have warned that radon is second only to cigarette smoking as a cause of lung cancer in the United States.

Radon gas is found throughout the world and in every state in America. In the United States, the average level inside homes (1.3 picoCuries per liter [pCi L^-1]) is about four times the average level outdoors (0.3 pCi L^-1). But restricted air flow and resulting gas accumulations can push indoor levels much higher. When indoor levels reach about three time the average (i.e., 4.0 pCi L^-1, the U.S. EPA action level), radon is considered a health threat.

Radon levels generally are highest in areas where soil is underlain by limestone. Evaluations of U.S. county data by EPA and the U.S. Geological Survey indicate that many counties in the Tennessee Valley region have a high potential for radon exposure in the home. Limestone-rich areas of Alabama, Tennessee, and Kentucky are, on average, the most vulnerable to radon, with the Coastal Plain and Delta areas of west Tennessee and Mississippi the least vulnerable. But the variability within all areas is quite high. That is why every home should be tested, regardless of geographic location.

Combustion Pollutants—Water vapor, carbon monoxide (CO), carbon dioxide (CO2), nitrous oxide (NO), nitrogen dioxide (NO2), and fine particles are emitted by combustion sources. Depending on the fuel source, hydrocarbon gases, organic particles, metals, and sulfur oxides also can be emitted.

Unvented combustion sources are a ubiquitous source of NO2 and CO in homes. The NO2 levels in homes with gas ranges are nearly always higher than the NO2 levels in homes with electric ranges. Carbon monoxide emissions from gas ranges typically are ten times higher than NO2 emissions. It is estimated that 45 percent of homes in the United States use natural gas.

Wood stoves and fireplaces emit NO2, CO, and fine particles. Emissions normally are vented outdoors, and only during startup and stoking are emissions likely to contaminate indoor air. Consequently, wood burning increases particulate levels indoors by only a few micrograms per cubic meter (µg/m3). However, the particulate matter from wood smoke has been found to be carcinogenic.

Kerosene heaters, in addition to emitting NO2, CO, and sulfur dioxide (SO2), also emit polycyclic aromatic hydrocarbons that are mutagenic. Approximately 11 percent of the U.S. population is exposed to gas or kerosene space heater emissions. Under certain conditions, exhaust gases from vented combustion appliances can enter the living space. For example, exhaust fans cause combustion emissions to be back-drafted into the living space. Combustion emissions also can leak into homes from roadways or, especially, from nearby attached or sublevel garages.

Tobacco Smoke—Tobacco smoke is probably the most studied indoor air pollutant. Over 50 compounds in tobacco smoke are either known or suspected carcinogens. Studies using electrostatic filters have revealed that, contrary to popular belief, most of the irritation and the odor from tobacco smoke comes from the gaseous pollutants and not the particulates. To place into perspective the issue of human exposure to Environmental Tobacco Smoke (ETS), a comparison was made of exposure to particulates originating from ETS and from coal-fired power plants. According to a 1993 U.S. EPA report, from a human exposure to particulates standpoint, a mere 2 percent reduction in environmental tobacco smoke would be equivalent to closing all coal-fired power plants in the United States.

Particles—Particles are very small solid or liquid substances that can become suspended in air. Particles can be organic or inorganic compounds, or they can be living or dormant organisms. Health effects from particle exposure depend on the types and concentrations of particles present, frequency and duration of exposure, and sensitivity of the individual. Of primary concern are the small respirable particles that can penetrate deep into the lungs and cause acute or chronic health effects. Tobacco smoke and other combustion products, cat dander, bacteria, viruses, asbestos, and outside air are the primary sources of fine particles (< 2 µm). Larger particles, such as mold, pollen, animal dander, and dust allergens do not penetrate as deeply but can cause allergic responses.

Formaldehyde—Formaldehyde is one of the most ubiquitous organic vapors in indoor air. Long before EPA recognized organic vapors as pollutants of indoor air, formaldehyde was known to be a pungent gas that irritated eyes and mucous membranes. It is found in hundreds of products, including medicines, cosmetics, toiletries, and food containers. Its most common use is in resins used to bind together laminated and chipped wood products. Formaldehyde is also used as a carrier for dyes in synthetic carpets. It is an irritant to the eyes, nose, and throat and has been linked to asthmatic symptoms. It also has been listed as a suspected human carcinogen.

Volatile Organic Compounds—Building furnishings, construction materials, consumer products, paints, adhesives, pesticides, and ETS are indoor sources of volatile organic compounds (VOCs). In an EPA study, more than 500 VOCs were identified in the air inside ten public access buildings. Emission rates from construction materials are difficult to predict because they generally decline after construction, having half lives of two to twenty weeks. VOC exposure levels are most influenced by the materials inside the home and the behavior of the occupants. With the exception of benzene, which is a human carcinogen, most prevalent VOCs are not carcinogenic; however, they can cause headaches, irritate the eyes, and reduce productivity of occupants.

Asbestos and Lead—Asbestos and lead from lead-based paints are two pollutants that pose the greatest health risk when they are dispersed in remodeling activities. Stringent control measures are required for renovation activities that involve asbestos. Lead in indoor environments poses the greatest risk to children. Asbestos and lead-based paints are still common in many buildings constructed prior to 1980.

Biological Agents—Biological agents include pollens from trees and plants and microbial cells such as viruses, bacteria, fungal spores, protozoans, algae, animal dander and excreta, and insect excreta and fragments. Mold hidden behind wall coverings and beneath carpet can emit microbial VOCs that can penetrate vapor barriers and enter the living space. Microbial growth most often occurs in bathrooms, damp basements, window casements, air conditioner cooling coils, and condensate drain pans. In general, moisture control in buildings is essential for preventing the growth of bacteria and fungi. Dust mites, a common allergen and cause of asthmatic symptoms among sensitive individuals, also can be controlled by reducing moisture.

Biological contaminants can cause or exacerbate respiratory diseases. Common disorders are hay fever or asthma. A more serious illness that can be caused by biological contaminants is hypersensitivity pneumonitis (farmer's lung disease, humidifier lung, and humidifier fever). Its symptoms are similar to pneumonia; however, the illness is the result of an immune response to an antigen and not an infection. If left untreated, hypersensitivity pneumonitis can cause permanent lung damage.

Indoor Air Pollution Control

Three methods for controlling indoor air pollution are ranked in terms of effectiveness as follows:

Eliminate pollutant source
Increase ventilation to dilute concentration of contaminant(s)
Filter or purify air

Radon, combustion pollutants, ETS, particles, formaldehyde, and VOCs all can be eliminated from indoor air. Radon gas can be vented from beneath slabs or crawl spaces. ETS can be eliminated by prohibiting smoking indoors. Exposure to other combustion emissions can be avoided by proper ventilation. Formaldehyde and VOC pollution in homes can be reduced by selecting building materials and furnishings that do not emit these pollutants and by choosing to use water soluble products in place of solvent-based products. Biological contaminants can be controlled by keeping relative humidity below 60 percent and by removing dust and other particles that can provide nutrients for microbes. Since mold can thrive on air-conditioner cooling coils and in condensate removal systems, it can be controlled by properly maintaining air-conditioners. Adequate insulation in the outer shell of homes, adequate circulation of air, and the use of storm windows can prevent condensation on windows and walls, eliminating sites for moisture-borne organisms.

Ventilation is effective for diluting the concentration of all indoor air pollutants. Exhaust ventilation can be used to remove moisture from kitchens and bathrooms and ETS from smoking areas. Where VOCs are emitted, ventilation can be used to dilute their concentration. Typically, outside air is pulled into homes when exhaust fans are operated. In homes that are airtight, mechanical ventilation may be required to provide adequate supplies of outside air. To keep bioeffluents and CO2 respired by occupants at acceptable levels, a minimum of 15 ft³/min of outside air per person is required.

Air filters and air purifiers use one of three types of filters: mechanical, electrostatic, and electronic. While effective in removing large particles, ordinary, flat mechanical filters are less effective in removing small, respirable particles than electrostatic filters or mechanical filters that are either pleated or have extended media. To effectively remove respirable particles in ETS, electronic filters that collect particles on oppositely charged plates are required.

Filters only remove airborne particles. Thus, particles settled on carpet, bedding, drapes and upholstery can be resuspended by occupants and settle out without reaching the filtration system. Filters also are ineffective in removing gases. And some filters that have been specifically designed to remove gases are not effective in removing all the gaseous pollutants typically found in indoor air. Like whole-house filters, air purifiers are only effective in removing ETS if they contain an electronic filter. Air purifiers also are limited by the volume of air they can handle and by how well that air is mixed. Air can be short circuited around the air handler, leaving pockets of air untreated. Ozone generators being sold as air purifiers are not recommended because they can cause indoor ozone levels to exceed the regulated limits for outdoor air.

Conclusions

Public awareness of the hazards associated with indoor air pollution and the high cost of energy required to maintain good indoor air quality have made it important for everyone concerned with construction, maintenance, management, and ownership of buildings to better understand indoor air pollution issues. Consumers can improve their indoor living environment by adopting a few simple IAQ pollution control measures. These measures not only will benefit personal health, but also will improve the integrity of their homes.

Information Contacts

Michael F. Broder
(256) 386-2475
mfbroder@tva.gov

William J. Parkhurst
(256) 386-2793
wjparkhurst@tva.gov

Last updated on 9-11-2002.
Inquiries and comments should be sent to wjparkhurst@tva.gov.