Animal agriculture is a $100 billion per year industry in the United States. Livestock and poultry are raised on an estimated 1.3 million farms throughout the nation. About 238,000 of these farms are considered animal feeding operations (AFO)—agriculture enterprises where animals are kept and raised in confinement. An estimated 95% of these are small businesses: most AFOs raise fewer than 300 animals. Very large AFOs, housing 300 or more animals such as cows (or equivalent numbers of other animal species), are defined as concentrated animal feeding operations, or CAFOs. For more than two decades, organizational changes within the industry to enhance economic efficiency have resulted in larger confined production facilities that often are geographically concentrated. Increased facility size, greater numbers of animals being raised at large feedlots, and regional concentration of livestock and poultry operations have, in turn, given rise to concerns over the management of animal wastes from these facilities and potential impacts on environmental quality. From an environmental quality standpoint, much of the public and policy interest in animal agriculture has focused on impacts on water resources, because animal waste, if not properly managed, can adversely impact water quality through surface runoff and erosion, direct discharges to surface waters, spills and other dry-weather discharges, and leaching into soil and groundwater. However, animal feeding operations can also result in emissions to the air of particles and gases such as ammonia, hydrogen sulfide, and volatile organic chemicals (VOC). At issue today are questions about the contribution of AFOs to total air pollution and corresponding ecological and possible public health effects. Resolving those questions is hindered by a lack of adequate, accurate, scientifically credible data on air emissions from AFOs, data that are needed to gauge possible adverse impacts and subsequent implementation of control measures. This report provides background on these issues. It first reviews the types of air emissions from livestock and poultry operations and their human health and environmental impacts. It then discusses provisions of several federal laws concerned with environmental impacts, beginning with the Clean Water Act, because protecting water resources has been the primary regulatory focus regarding livestock and animal operations. The Environmental Protection Agency (EPA) has authority to address AFO air emissions under several laws—the Clean Air Act; the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, or Superfund); and the Emergency Planning and Community Right-to-Know Act (EPCRA)—which are discussed next. Questions about the applicability of these laws to livestock and poultry operations have been controversial in several arenas and have drawn congressional attention. Agricultural emissions of greenhouse gases that have been of interest in connection with proposals to address the global challenge of climate change are discussed. Studies by the National Research Council concerning air emissions are reviewed, as are relevant activities of the states and the U.S. Department of Agriculture. Finally, the report identifies a number of key research questions needed to characterize and evaluate animal agriculture emissions. AFOs can affect air quality through emissions of gases (ammonia and hydrogen sulfide), particulate matter (PM), volatile organic compounds (VOC), hazardous air pollutants, microorganisms, and odor. AFOs also produce gases (carbon dioxide and methane) that are associated with climate change. The generation rates of odor, manure, gases, particulates, and other constituents vary with weather, time, animal species, type of housing, manure handling system, feed type, and management system (storage, handling, and stabilization). Emission sources include barns, feedlot surfaces, manure storage and treatment units, silage piles, animal composting structures, and other smaller sources, but air emissions come mostly from the microbial breakdown of manure stored in pits or lagoons and spread on fields. Each emission source will have a different profile of substances emitted, with rates that fluctuate through the day and the year. The sources, fate, and transport of AFO emissions are illustrated in Figure 1 . Pollutants associated with AFOs have a number of environmental and human health impacts. Most of the concern with possible health effects focuses on ammonia, hydrogen sulfide, and particulate matter, while major ecological effects are associated with ammonia, particulates, methane, and oxides of nitrogen. The nitrogen in animal manure can be converted to ammonia (NH 3 ) by a combination of processes. Ammonia released from the surface of liquid manure storage structures rapidly adheres to particles in the air, due to its cohesive properties, thus contributing to the formation of ambient particulate matter, specifically ammonium nitrate and ammonium sulfate. These particles form to a varying degree in the presence of ammonia and oxides of nitrogen or sulfur (see below). Once emitted, ammonia also is re-deposited back to earth in rainfall that can harm surface waters and aquatic life in lakes and streams. Ammonia aerosols in rainfall contribute to oxygen depletion of aquatic systems and excessive growth of algae, as well as acidification of the environment. It is estimated that emissions from animal waste account for about one-half of the total natural and anthropogenic ammonia emitted in the United States annually. Ammonia has a strong, sharp, characteristic odor that disperses rapidly in the air. Health effects at low concentrations include eye, nose, and throat irritation; exposure at very high short-term concentrations can be lethal. Particles are highly complex in size, physical properties, and composition. For regulatory purposes, airborne particulate matter (PM) is commonly considered as coarse particles (those less than 10 microns in diameter, referred to as PM 10 ), or fine particles, those less than 2.5 microns in diameter (referred to as PM 2.5 ). PM 10 and PM 2.5 can be directly emitted geologic material, including from unpaved roads and other dust. Agriculture is a major direct source of PM 10 , from sources such as grain mills or storage facilities, feeding equipment, and particles generated in other mechanical processes. In contrast, PM 2.5 is a different class of particles, resulting more from evaporation and atmospheric chemical processes than from direct emissions. Fine particles are formed in the atmosphere through the chemical interaction of precursor emissions such as sulfur oxides, nitrogen oxides, and VOCs. AFOs can contribute directly to particulate matter through several mechanisms, including animal activity, animal housing ventilation units, and particles of mineral and organic material from soil and manure that adhere to air molecules. As described above, particulate matter can contribute indirectly to fine particle formation by emissions of ammonia, nitrogen oxides, and hydrogen sulfide, which are converted to aerosols through reactions in the atmosphere. Particle formation is highly dependent on atmospheric temperature, humidity, concentrations of the precursor compounds, and other factors, so the particle formation is variable and difficult to predict. Particles of differing sizes have been linked to health effects. Larger particles tend to be deposited in the upper airways of the respiratory tract, whereas small particles have both health and environmental effects: they can be deposited in the smallest airways in the lungs and, while still airborne, also play an important role in formation of regional haze. Populations with long-term exposure to heavier loads of particles have higher rates of mortality from major cardiovascular diseases, as well as increased rates of morbidity. The primary environmental and ecological effects of particles are related to haze and decreased visibility, which is caused by the suspended aerosols that both absorb and scatter light. Hydrogen sulfide (H 2 S) is a colorless gas with a strong and generally objectionable rotten egg odor. It is produced in anaerobic (oxygen-deprived) environments from the microbial reduction of sulfate in water and the decomposition of sulfur-containing organic matter in manure. Acute human health effects include respiratory and cardiovascular irritation, as well as headaches. H 2 S may have local effects of concern—especially odor—and may contribute to the atmospheric sulfur burden of regions with a high density of AFOs, but few other sources. Methane and nitrous oxide are greenhouse gases that are known to contribute to global warming. An estimated one-half of global methane comes from manmade sources, of which agriculture is the largest source, with livestock production being a major component within the sector. EPA estimates that more than 30% of the nation's methane emissions come from livestock. Agricultural methane is produced by ruminant animals, but also is emitted during microbial degradation of organic matter under anaerobic conditions. Nitrous oxide forms via the microbial processes of nitrification and denitrification. In the United States, manure management accounts for about 5% of nitrous oxide emissions and 7.5% of methane emissions. (See " Agricultural Emissions of Greenhouse Gases ," below, for discussion.) Many of the complaints about AFOs are generated by odor . Odor from AFOs is not caused by a single substance, but is rather the result of a large number of contributing compounds, including ammonia, VOCs, and hydrogen sulfide. As classes of compounds, odor and VOCs can be considered together. VOCs (also referred to as reactive organic compounds, or ROG) vaporize easily at room temperature and include a large number of constituents, such as volatile fatty acids, sulfides, amines, alcohols, hydrocarbons, and halocarbons. In terms of their health and environmental effects, some VOCs may irritate the skin, eyes, nose, and throat. They also can be precursors to the formation of PM 2.5 and ozone (smog). Adverse effects of ozone include lung damage and exacerbated respiratory disease, as well as diminished visibility. Ozone in the troposphere, the lowest layer of the atmosphere which is closest to the Earth, has both natural and anthropogenic sources. It can damage forests, crops, and manmade materials, and harm respiratory tissue through inhalation. Ozone that occurs naturally at ground-level is generally at low concentrations that are not believed to threaten human health or the environment.