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Urban Natural Resources Stewardship

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Tree Influences on Human Exposure to Ultraviolet Radiation

[photo:] Instruments to measure ultraviolet and visible solar radiation.  Here the sensors are above tree canopies for comparison to measurements below canopy.Research Issue

By influencing solar radiation, trees and forests affect the exposure of other living things, including people, to the Sun’s rays. The tree influences and the effects on people, animals, and plants vary with the wavelength of the solar radiation. The shortest wavelengths of solar radiation to reach to the surface of Earth are those of the “ultraviolet” wavebands. Though we can’t seem them, ultraviolet (UV) wavelengths have important effects on life. The effects include creating vitamin D in human skin and thus being beneficial to health, but also, with excess exposure, causing sunburn, skin cancer, and cataracts of the eye.

Tree shade can help people enjoy the out-of-doors while avoiding excess UV exposure, but more information is needed on just how trees modify the potential exposure. An important question is the radiation spectrum in tree shade and near trees, because medical research shows the relative beneficial versus detrimental effects of UV radiation on people vary with the spectrum of received radiation. Tree influences on UV radiation are difficult to evaluate and describe because UV radiation is widely scattered across the sky so that UV shade patterns differ greatly from shade patterns of the visible radiation that we can see. Also, although UV radiation is highly active in influences living things, it is much more difficult to measure than visible solar radiation.

Our Research

Working with cooperators, we have carried out a series of studies to measure and model the influence of trees on UV radiation in their vicinity.  The trees included individuals and trees in groups, such as are often found in urban areas, and deciduous trees with and without leaves.  Our research also investigated the factors that affect tree influences on UV—the distribution of scattered solar radiation from the sky, the effects of clouds, the above-canopy radiation in a city as compared to rural areas, and the reflectivity of leaves.  The tree influence models led to preliminary estimates of tree influences on average UV exposure of pedestrians in different land-use types, and our future research aims to refine those estimates.  

Expected Outcomes

We expect our research results on UV radiation to help environmental and health agencies recommend human behavior for people to acquire appropriate exposure to UV radiation.  At present most health agencies recommend minimizing UV radiation exposure, though some suggest moderate exposure may be beneficial for the benefits of vitamin D.  All health authorities urge avoidance of so much exposure to UV radiation that sunburn occurs.

Research Results

Grant, R.H., Heisler, G.M., 2006. Effect of cloud cover on UVB exposure under tree canopies: Will climate change affect UVB exposure? Photochem. Photobiol. 82, 487-494.

Heisler, G.M., 2005. Health impacts of ultraviolet radiation in urban ecosystems: a review (Invited). In: Bernhard, G., Slusser, J.R., Herman, J.R., Gao, W. (Eds.), Ultraviolet Ground- and Space-based Measurements, Models, and Effects V. SPIE--The International Society for Optical Engineering, San Diego, CA, pp. 5886M-5881-5817.

Heisler, G.M., Grant, R.H., Gao, W., Slusser, J.R., 2004. Solar ultraviolet-B radiation in urban environments: the case of Baltimore, MD. Photochem. Photobiol. 80, 422-428.

Grant, R.H., Heisler, G.M., Gao, W., Jenks, M., 2003. Ultraviolet leaf reflectance of common urban trees and the prediction of reflectance from leaf surface characteristics. Agric. Forest Meteorol. 120, 127-139.

Heisler, G.M., Grant, R.H., Gao, W., 2003. Individual- and scattered-tree influences on ultraviolet irradiance. Agric. Forest Meteorol. 120, 113-126.

Qi, Y., Bai, S., Heisler, G.M., 2003. Changes in ultraviolet-B and visible optical properties and absorbing pigment concentrations in pecan leaves during a growing season. Agric. Forest Meteorol. 120, 229-240.

Gao, W., Grant, R.H., Heisler, G.M., Slusser, J.R., 2002. A geometric ultraviolet-B radiation transfer model applied to vegetation canopies. Agronomy J. 94, 475-482.

Grant, R.H., Heisler, G.M., 2001. Multi-waveband solar irradiance on tree-shaded vertical and horizontal surfaces: cloud-free and partly cloudy skies. Photochem. Photobiol. 73, 24-31.

Grant, R.H., Heisler, G.M., 2000. Estimation of ultraviolet-B irradiance under variable cloud conditions. J. Appl. Meteorol. 39, 904-916.

Heisler, G.M., Grant, R.H., 2000. Ultraviolet Radiation, Human Health, and the Urban Forest. U.S. Department of Agriculture, Newtown Square, PA, General Technical Report pp. 35.
Heisler, G.M., Grant, R.H., 2000. Ultraviolet radiation in urban ecosystems with consideration of effects on human health. Urban Ecosys. 4, 193-229.

Grant, R.H., Heisler, G.M., Gao, W., 1997. Clear sky radiance distributions in ultraviolet wavelength bands. Theor. Appl. Climatol. 56, 123-135.

Grant, R.H., Heisler, G.M., 1997. Obscured overcast sky radiance distributions for the UV and PAR wavebands. J. Appl. Meteorol. 56, 1336-1345.

Grant, R.H., Heisler, G.M., Gao, W., 1997. Ultraviolet sky radiance distributions of translucent overcast skies. Theor. Appl. Climatol. 58, 129-139.

Grant, R.H., Heisler, G.M., Gao, W., 1996. Photosynthetically-active radiation: sky radiance distributions under clear and overcast conditions. Agric. Forest Meteorol. 82, 267-292.

Grant, R.H., Heisler, G.M., 1996. Solar ultraviolet-B and photosynthetically active irradiance in the urban sub-canopy: a survey of influences. Int. J. Biometeorol. 39, 201-212.

Research Participants

Principal Investigator

Research Partners

  • Wei Gao, USDA UVB Radiation Monitoring and Research Program
  • Richard H. Grant, Purdue University
  • James Slusser, USDA UVB Radiation Monitoring and Research Program
  • Yadong Qi, Southern University


Last Modified: 10/03/2019