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Reduced Risk

cite: Wolf, K.L., S. Krueger, and K. Flora. 2015. Reduced Risk - A Literature Review. In: Green Cities: Good Health ( College of the Environment, University of Washington.


The field of environmental health has historically addressed the risks of toxins, pollutants, and unfavorable environments within communities. There is now an emerging interest in the positive health contributions of environment and in providing quality, well-designed parks and nature for public health benefits. Trees and vegetation can dampen ambient noise, improve air quality, cool over-heated urban centers, and be a food security solution.

Fast Facts

Urban Stressors

The density of life in cities (of buildings and other people) can be the source of stressors that have effects on community and individual health. A list of stressors follows, and some of the research about elements of metro nature that contribute to reductions or mitigations of stressors.

Air Quality

The 1970 Clean Air Act identifies seven air pollutants considered harmful to human health and the environment including ground-level ozone, lead, carbon monoxide (CO), nitrous dioxide (NO2), sulfur dioxide (SO2), volatile organic compounds (VOCs) and particulate matter.1 Ground-level ozone is the most conspicuous pollutant, and is responsible for more than 95% of all days in the U.S. that violate air quality standards. Most detrimental to children and those frequently active outdoors, ozone inflames the lungs and increases susceptibility and aggravation of respiratory diseases and infections.1 A number of studies have linked ambient levels of ultrafine particles to short-term, adverse health effects,2,3 such as significant changes in indices of inflammation, homeostasis, lipids, and cardiovascular function.4

Vegetation, and trees in particular, can act as natural filters for both gases and particulate matter in urban environments.5 Trees capture particulate matter on their leaf surfaces, though the particulates may become re-suspended due to wind or rain or deposited on the ground.6,7 The effects of urban trees on fine particulate matter (PM2.5) was modeled for 10 U.S. cities, with total annual PM2.5 removal varying from 5.2 tons in Syracuse to 71.1 tons in Atlanta. The estimated mortality reduction was approximately 1 person per year for each city but was as high as 7.6 people per year for New York. Overall air quality improvements attributed to urban trees ranged between 0.05% in San Francisco to 0.24% in Atlanta.8

In addition, in areas with complete tree cover, trees can remove as much as 15% of the ozone, 14% of the SO2, 8% of the NO2, and 0.05% of the CO from the air.5 Researchers using advanced modeling that accounts for a variety of landscape factors estimate that the NO2 reduction associated with Portland, Oregon’s urban trees results in significantly fewer respiratory problems, valued at $7 million in health care savings annually.9

Pollution removal rates differ depending on the amount of air pollution; length of time trees retain their leaves; species, size, age and location; precipitation; and other meteorological conditions.5) A large healthy tree (greater than 30 inches in diameter) removes about 70 times more air pollution annually than a small healthy tree (those less than 3 inches in diameter).10 Through helping to cool urban areas and mitigate the urban heat island effect, green spaces also help address air pollution by reducing the formation of photochemical ozone.11 Shade provided by urban trees also reduces energy demand, indirectly contributing to improved air quality.12

Some species like pine, larch, and silver birch have a more positive effect on air quality than those like oak, willow, and poplar, because they emit lower levels of VOCs , which contribute to the formation of other pollutants, such as ozone and particulates.11 Trees can restrict air movement in dense urban areas and prevent the localized distribution of pollutants.13 Appropriate species selection and thoughtful planting design can maximize beneficial effects on air quality.6,7

Asthma and Allergies

Pollen is associated with allergies and asthma.14 Urban trees are often selected for hardiness, ability to withstand pollution and resistance to disease, and tidiness fruit and leaf litter. However, many popular ornamental and non-fruit bearing species produce excessive pollen, perhaps aggravating allergies.sup>14 Pollen production varies among species. The male trees of dioecious species (those with separate sexes) are heavy pollinators. Male trees may be preferred for urban settings because they do not produce messy fruits, however they should be avoided due to allergy concerns. Some tree species are also shown to produce higher levels of pollen as concentrations of CO2 gas increase.sup>15 The allergenic effects of tree pollen may be enhanced by the presence of other air pollutants.16

Childhood asthma in the U.S. increased by 50% from 1980 to 2000, with higher prevalence in poor urban communities and substantial racial, ethnic, and socio-economic disparities.17,18 Research suggests the causes of this drastic increase might be changes in the environment (indoors and outdoors) and changes in lifestyle.19,20,21 Exposure to air pollution could be one contributing factor causing the excess asthma in urban areas.22,23,24 In a study of data on street tree density and asthma rates in New York City, the results show that street trees were associated with lower asthma rates.25 After adjustments to take density rates and proximity to sources of air pollution into account, an increase of 132 trees/mi2 was associated with a 29% lower early childhood prevalence of asthma.25 On the other hand, studies have indicated a higher prevalence of tree pollen allergies among children with greater tree canopy cover at their prenatal address. 25

Reduced Physical Activity

The Centers for Disease Control reported in 2008 that 25.4% of American adults were not physically active at all.26 Inactivity is contributing to the recent upsurge in obesity rates and associated diseases in the U.S. and other industrialized nations.27 Obesity can lead to increased risk for a number of serious physical and mental health conditions, including congestive heart failure, diabetes, osteoarthritis and coronary heart disease.28 Routine physical activity can help reduce these health risks, and also improve joint structure and function, muscle strength,29 relieve symptoms of depression and anxiety, improve mood, and enhance physical and psychological well-being.29

Research findings suggest that appealing and easily accessible green environments may motivate and encourage physical activity. The presence of nature can affect people’s perceptions and motivations about activity; one study found that in less vegetated neighborhoods, walking distances were judged as farther than they actually were, and residents did fewer trips by foot.30

Activity in outdoor green spaces – at any level, intensity, duration, or type – has been associated with mental and physical benefits.31,32 People who use parks and open spaces are three times more likely to achieve recommended levels of physical activity than non-users.33 In European studies of residential areas, residents with high levels of neighborhood green space were three times more likely to be physically active and their likelihood of being overweight or obese was reduced by up to 40%.34 A study of elderly residents that had nearby parks, tree-lined streets, and space for taking walks showed higher longevity over a 5-year study period.35


The amount and character of noise - or the acoustic landscape - is very important to human health and well-being.36 On one hand, music may be used to improve moods and satisfaction in the workplace and life. Some people seek quiet or relaxing sounds of nature to recover from a stressful day or simply to relax and decompress. However, sound has also been used in warfare; sonic cannons produce a deafening aural assault used to break resolve and willpower.37

In urban environments quiet can be difficult to find. A study by the MORI Institute in the U.K. found that 63% of people experience noise from neighbors and about half the reports were of annoyance.38 Even moderate sound levels can cause feelings of disturbance, increased blood pressure,40 increased risk of heart attacks,39 decreased well-being and quality of life,41,42 and stress reactions and sleeplessness.39 A long-term study of 18,973 individuals in the Netherlands found that individuals who experienced a cardiovascular health event during a 13 year period had on average higher exposure to road traffic day and night and exposure to air pollution at home.43

Sound can impacts our quality of life and mental health. In one study those with access to a quiet side of their home, not exposed to traffic or disruptive noises, had lower stress-related psychosocial symptoms and improved sleeping patterns than those always in a noisy environment.44 Noise can also make it more difficult to learn and apply cognitive thinking skills.39,45 Green space, particularly trees and large shrubs, can reduce ambient noise by providing a barrier or screen.11 Vegetation can be used to dampen and reduce noise, by providing a noise barrier as well as softening sharp tones.46,11 For example, residents of neighborhoods with parks and green spaces show significantly lower dissatisfaction and disturbance with traffic noises.47,48

Many factors affect vegetation’s ability to reduce noise: the biomass distribution, plant spacing, plant size, leaf size and shape, and the ability of the plant material to obstruct or absorb noise.49 In order to effectively buffer noise by 3-8 decibels, a planting strip must generally be over 10m wide.50 Research suggests that dense planting reaching to the ground and with no gaps may achieve noise reductions of up to 15 decibels.51 The degree of noise reduction can partially be perception; qualitative studies found that when portions of a vegetative barrier were removed listeners perceived that the volume level increased disproportionately.52

Vegetation can also affect perceptions of sound, which may also contribute to health. In a study of perceptions about soundscapes researchers compared subjective ratings to objective decibel measurements.53 They found that people rate man-made and natural sound differently, preferring the natural even if the sound levels are high. In a similar study, participants rated loud biological noises (wind, water, birds) as desirable, suggesting that a tranquil experience can be engineered within urban environments.54

Urban Lifestyles and Stress

Urban environments and busy lifestyles can be stressful. Stress can affect one’s well-being and lead to various ailments as well as anxiety-related disorders. Visual stimuli and positive aesthetics, such as the presence of plants, can be used to relieve tension and reduce stress.55,56 For example, increased greenery along transportation routes, such as major commuting roads, could be used to reduce driver stress, by reducing noise levels and providing restorative, natural scenery.57 Nature sounds have been shown to reduce stress. In a study comparing nature sounds versus various noise levels in the recovery from sympathetic arousal (a response to stress), nature noises were perceived as more pleasant and resulted in the fastest recovery from a psychological stressor (Figure 1).58

<<<<<FIGURE 1 HERE>>>>


Recent shifts in both global and regional climate are contributing to increased environmental risks and health hazards. In the past century, the climate has shown marked changes, including increased average global temperature; changes to rainfall patterns; increased levels of CO2 , methane, and other gases; as well as many other direct and indirect effects.59 These changes impact human health, and the populations in urban environments are often more vulnerable to climatic changes due to residential density.59

Urban Heat Island

Higher temperatures can lead to increased discomfort, respiratory difficulties, heat cramps, exhaustion, non-fatal heat stroke, and heat related mortality, especially during heat waves.60 The Center for Disease Control estimates that between 1979 to 1999 exposure to excessive heat contributed to over 8,000 premature deaths in the U.S.61 A recent study found that for each 2°F increase in temperature in the U.S. there are an estimated 20-30 excess cancer cases and roughly 1,000 excess air-pollution-associated deaths.62 Densely-populated neighborhoods with high concentrations of ethnic/racial minorities appear to bear an inequitable amount of the risk associated with the urban heat island effect.63

The urban heat island effect and high density makes cities particularly vulnerable to heat waves.64 During a 2006 heat wave in California, emergency department visits showed significant increases in acute kidney failure, kidney inflammation, diabetes, cardiovascular diseases, and electrolyte imbalances.65

Urban development affects the absorption, reflection, and retainment of solar radiation as well as water retention and cooling, creating the urban heat island effect.60 Most urban building materials have high heat capacities - they store heat effectively. As a result, metropolitan areas can absorb and store twice the amount of heat during the day as rural areas66 and then release thermal heat at night.67 Higher temperatures can increase energy demand and increase levels of air pollution and greenhouse gas emissions.

Paved and built impervious surfaces in urban areas - like concrete and roofs - result in less shade and moisture retention to cool the urban environment.60 Many steps can be taken in cities to ameliorate the effects of a heat island and decrease the susceptibility of cities to heat waves. For example, green roofs, cool pavement, increased vegetation, trees, and permeable surfaces can be used to reduce the heat island effect.60

The shading effect of trees as well as the cooling effects of evapotranspiration from leaves of vegetation reduces air temperatures.68,69,70 A meta-analysis across studies found that on average, parks are almost 2°F cooler than the surrounding urban areas during the day71 (Figure 2)60. Researchers have found a correlation between shading in parks and higher visitation levels in warm climates, suggesting that shade is an important element of park design.72 A park’s cooling effect may extend into adjacent urban areas. Researchers estimate that cooling effects extend from 115 to 2,788 feet away from parks in Beijing, depending on the character of the area around each park.73 A study in Athens, Greece attributed wind speed as the most important factor extending cooling effects74, suggesting that city design that optimizes airflow around parks may aid ambient cooling.


Extreme heat is a major source of weather-related mortality and illness around the world. In subtropical Australia, a 50°F increase in daily maximum temperature during the summer was associated with a 7.2% increase in hospital admissions the following day.75 Simulations of urban vegetation schemes in Melbourne, Australia suggest that average summer temperatures can be reduced by 0.5-2°C by doubling the city’s vegetation coverage and transforming much of the urban core into suburbs and parklands.76

Health Risk Inequity

Parks and vegetation can also have healing and restorative properties that can decrease health risks. However, many studies indicate that there are racial, ethnic, and socio-economic disparities in access to and quality of neighborhood green space.75 Those in a lower socioeconomic position are often less likely to be active than those with high socioeconomic status, in part because their neighborhood environments are often less conducive to physical activity.76 The stress of poverty can cause physiological responses, which are expressed as increased risk of various diseases, including heart disease.77,78 The presence of nearby green space may reduce physiological and perceived stress among those living in poorer, urban neighborhoods.79,80 Research also suggests that health inequalities related to income deprivation for all-cause mortality and mortality from circulatory disease are lower in populations living in the greenest areas.81 Neighborhood green space may also improve people’s perceptions of their own health (See Figure 3).82


Food Security

Reliable access to nutrient-rich food, known as food security, is directly correlated to one’s physical and mental health and socioeconomic status.83,84 Those who experience chronic limited access to food, or reliance on begging or scavenging, face an increased risk of obesity by 20-40%,85,86 due mainly to poor nutrition. Other factors in food security disparities include “food deserts” in low income neighborhoods, where fast food outlets are more common than healthy food options87, an over-reliance on cheap, energy dense foods high in fat and sugar88, and episodic food shortages.87,89


Urban gardens can act as safety nets for low-income communities and at risk populations by bolstering food security, providing fresh produce, and setting up opportunities for physical activity.

Gardens appear to increase people's consumption of fresh produce. One study showed that respondents were 3.5 times more likely to consume at least five servings of fruit or vegetables if they or a family member had participated in a community gardening project within the last 12 months.90 Similar research finds that 56% of community gardeners met national recommendations for fruit and vegetable consumption, compared with 37% of home gardeners and 25% of non-gardeners.91 The length of time that an individual reports having participated in gardening does not appear to have a relationship with the amount of produce consumed, suggesting that gardening programs targeted toward older individuals can effectively increase fruit and vegetable consumption for elders.92

Community or individual gardeners often employ ecological agricultural practices that do not rely heavily on chemical fertilizers and pesticides, thereby reducing pollution and potential threats to public health.93 Community gardens also create social benefits, fostering interpersonal relationships among neighbors.94 Community and individual gardens offer sources for improved nutrition; a stabilized, secure food source; and a reduction in the carbon footprint and environmental risks associated with food supply.

Urban Foraging

Urban forests can provide opportunities for residents to harvest wild plants and fungi for food, medicine and to maintain cultural traditions.95 Aside from native edible plants, urban forests often contain exotic fruit and nut-bearing trees that provide sustenance for community members. 97 Increasingly, non-profit organizations and municipal governments are promoting the use of urban green space for publicly-accessible edible landscaping and orchards to improve food security and environmental sustainability.96

Cleaning Contaminated Soils

Environmental risk literature indicates that heavy metals and toxins in urban soils and waters can contaminate gardens and wild foods, but that actual toxin uptake varies greatly by species.97 Phytoremediation, the use of certain plants to treat soil pollutants such as heavy metals, hydrocarbons, and oils, can help prepare contaminated areas for safer urban gardening or forage. Quick-growing mustard plants may effectively accumulate lead that entered urban environments through historical use of lead-based paints and gasoline additives98. Sunflowers are shown to accumulate both lead nitrate and zinc nitrate from contaminated roadsides.99 Experiments with poplars demonstrated that the trees successfully removed 19 of 29 potential PCBs from contaminated garden soil within 96 growing days.100 Additional plants commonly used in phytoremediation include willows, vetiver grass, reeds, cattails, and pennycress. These plants not only accumulate or alter toxins but also help rebuild soil structure at heavily impacted sites.101

Additional Risk Reduction

Urban Ecosystem Services

Trees, parks, open spaces and engineered nature are part of urban ecosystems, and provide ecosystem services. They can help to purify water, improve air quality, mitigate flooding and droughts, aid soil fertility and erosion control, reduce summer-time air temperatures and ultraviolet radiation, and bind toxic substances.102 Landscape elements like bioswales and rain gardens are used to filter and improve surface run-off water by removing silt and pollution, and reduce storm water.103


Urban trees may also play a role in reducing traffic hazards. The most recent research suggests that trees may improve driving safety. One study found a 46% decrease in crash rates across urban arterial and highway sites after landscape improvements were installed104. Another study found that placing trees and planters in urban arterial roadsides reduced mid-block crashes by 5% to 20%.105 Overall the presence of trees and roadside vegetation may not only calm traffic but also reduce the stress of those behind the wheel106 , making transportation corridors safer for drivers, pedestrians and bicyclists.

Project support was provided by the national Urban and Community Forestry program of the USDA Forest Service, State and Private Forestry.


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