Tree and forest effects on air quality and human health in the United States
Introduction
Air pollution is a significant problem in the United States that affects human health and well-being, ecosystem health, crops, climate, visibility and man-made materials. The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards for six “criteria pollutants” – that are both common throughout the United States and detrimental to human welfare (US EPA, 2013a). These pollutants are: carbon monoxide (CO), nitrogen dioxide (NO2), ozone (O3), lead (Pb), sulfur dioxide (SO2), and particulate matter (PM), which includes particulate matter less than 10 microns (PM10) and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter. Health effects related to air pollution include impacts on pulmonary, cardiac, vascular, and neurological systems (e.g., Pope et al., 2002). In the United States, approximately 130,000 PM2.5-related deaths and 4700 O3-related deaths in 2005 were attributed to air pollution (Fann et al., 2012).
Trees and forests, like air pollution, vary throughout the United States (e.g., percent tree cover, species composition). Trees affect air quality through the direct removal of air pollutants, altering local microclimates and building energy use, and through the emission of volatile organic compounds (VOCs), which can contribute to O3 and PM2.5 formation (e.g., Chameides et al., 1988). However, integrative studies have revealed that trees, particularly low VOC emitting species, can be a viable strategy to help reduce urban O3 levels (e.g., Taha, 1996, Nowak et al., 2000).
Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. For O3, SO2 and NO2, most of the pollution is removed via leaf stomata. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Trees directly affect particulate matter in the atmosphere by intercepting particles, emitting particles (e.g., pollen) and resuspension of particles captured on the plant surface. Some particles can be absorbed into the tree, though most intercepted particles are retained on the plant surface. The intercepted particles often are resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall. During dry periods, particles are constantly intercepted and resuspended, in part, dependent upon wind speed. The accumulation of particles on the leaves can affect photosynthesis (e.g., Darley, 1971) and therefore potentially affect pollution removal by trees. During precipitation, particles can be washed off and either dissolved or transferred to the soil. Consequently, vegetation is only a temporary retention site for many atmospheric particles, where particles are eventually moved back to the atmosphere or moved to the soil. Pollution removal by urban trees in the United States has been estimated at 711,000 tonnes (t) per year (Nowak et al., 2006a).
While various studies have estimated pollution removal by trees (e.g., Nowak et al., 2006a, McDonald et al., 2007, Tallis et al., 2011), most studies on pollution removal do not directly link the removal with improved human health effects and associated health values. A few studies that have linked removal and health effects include one in London where a 10 × 10 km grid with 25% tree cover was estimated to remove 90.4 t of PM10 annually, which equated to the avoidance of 2 deaths and 2 hospital admissions per year (Tiwary et al., 2009). In addition, Nowak et al. (2013) reported that the total amount of PM2.5 removed annually by trees in 10 U.S. cities in 2010 varied from 4.7 t in Syracuse to 64.5 t in Atlanta. Estimates of the annual monetary value of human health effects associated with PM2.5 removal in these same cities (e.g., changes in mortality, hospital admissions, respiratory symptoms) ranged from $1.1 million in Syracuse to $60.1 million in New York City. Mortality avoided was typically around 1 person yr−1 per city, but was as high as 7.6 people yr−1 in New York City.
Tree cover in the United States is estimated at 34.2 percent and varies from 2.6 percent in North Dakota to 88.9 percent in New Hampshire (Nowak and Greenfield, 2012). As people and trees exist throughout a landscape in varying densities, not only will pollution removal and its effects on local pollution concentrations vary, but so will the associated human health impacts and values. The objectives of this paper are to estimate the amount of air pollution (NO2, O3, PM2.5, SO2) permanently removed by trees and forests within urban and rural areas of the conterminous United States in 2010, and its associated monetary value and impact on human health.
Section snippets
Methods
To estimate avoided health impacts and associated dollar benefits of air pollution removal by trees and forests in the conterminous United States in 2010, four types of analyses were conducted. These analyses were conducted at the county-level for all urban and rural areas to estimate: 1) the total tree cover and leaf area index on a daily basis, 2) the hourly flux of pollutants to and from the leaves, 3) the effects of hourly pollution removal on pollutant concentration in the atmosphere, and
Results
The total amount of pollution removal in 2010 by trees and forests in the conterminous United States was 17.4 million t (range: 9.0 million t to 23.2 million t), with a human health value of $6.8 billion (range: $1.5 billion to $13.0 billion) (Table 2). The range in values is based on the typical range of deposition velocities, but other uncertainties based on input data (e.g., tree cover, pollution concentration) and modeling of health benefits would increase the range, but the value of these
Discussion
Pollution removal by trees and forests in the United States is substantial at more than 17 million t removed in 2010. As 96.4 percent of the conterminous United States is rural land and percent tree cover is comparable between urban and rural land (Nowak and Greenfield, 2012), 96.3 percent of pollution removal from trees occurred on rural land. However, as human populations are concentrated in urban areas, the health effects and values derived from pollution removal are concentrated in urban
Conclusion
Modeling broad-scale effects of pollution removal by trees on air pollution concentrations and human health reveals that while the percent reduction in pollution concentration averages less than one percent, trees remove substantial amounts of pollution and can produce substantial health benefits and monetary values across the nation, with most of the health values derived from urban trees.
Acknowledgments
Funding for this project was provided in part by the U.S. Forest Service's RPA Assessment Staff and State & Private Forestry's Urban and Community Forestry Program. We thank Laura Jackson and Shawn Landry for comments on previous versions of this manuscript and John Stanovick for statistical assistance and review.
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