 |
|
|
Urban Heat IslandsOn a warm summer day, the air in a city can be 6-8 degrees Fahrenheit hotter than surrounding areas with trees. Trees intercept solar radiation and transpire water. When the effects of this evapotranspiration are combined with the effect of strategically placed shade trees, temperatures can drop by as much as 9degrees Fahrenheit . Cities without adequate tree cover create "urban heat islands." Tom Kimmerer of TreeGuide.com Discusses OzoneThere needs to be a distinction made between tropospheric ozone (=ground level ozone = 'bad' ozone) and stratospheric ozone (=the ozone layer = 'good' ozone). I am not aware of any evidence that tropospheric ozone is linked to cancer. There is no question that it exacerbates respiratory problems in susceptible individuals (those with asthma, emphysema, the elderly, the very young), but no solid link to carcinogenesis in humans. Stratospheric ozone depletion (i.e the loss of ozone) is linked to an increase in skin cancer in humans. The urban heat island effect increase tropospheric ozone concentrations. Tropospheric ozone is a secondary pollutant created by the interaction of nitrogen oxides, hydrocarbons, and sunlight. Since its creation depends on a chemical reaction, and most chemical reactions are temperature dependent, hotter cities will have more ozone. Reducing the urban heat island effect will consequently reduce ozone concentrations within the island (Click here for visual of heatisland in Atlanta). The public health benefits of reducing the urban heat island effect are substantial, and include a reduction in respiratory illness due to ozone inhalation. A reduction in ozone will not, to my knowledge, reduce the incidence of cancer by any known mechanism. The National Toxicology Program concluded that "no conclusive evidence exists to link ozone exposure to lung cancer in humans... three of four animal studies conducted between 1985 and 1993 that are cited by the NTP revealed no increase in lung neoplasms from ozone exposure, and the findings about ozone's promotion effects were unclear." The National Cancer Institute states that the jury is still out on the question of long-term ozone exposure and carcinogenicity, but that at present there is no compelling evidence of carcinogenicity. The best argument in favor of trees in the urban landscape is that they reduce the urban heat island effect in a measurable way. Reducing urban temperatures reduces ozone concentrations, and this has important public health and economic consequences. With new ozone standards set to go into effect, many urban areas will now be looking for ways to reduce ozone concentrations. Trees provide a low-cost way to come into compliance, provided species are carefully selected to avoid those that emit large amounts of hydrocarbons. Tom Kimmerer, PhD |