Stratospheric Ozone and Human Health

In the unit on human health, we will discuss in more detail the reason stratospheric ozone is directly important to humans, but at this point we will briefly discuss the relationship of stratospheric ozone to ultraviolet (UV) light.

Wavelengths of light in the visible part of the spectrum of solar energy extend from about 0.7 to about 0.4 nanometers. Radiant energy having wavelengths shorter than about 0.4 nanometers is classified as ultraviolet light: "near UV" or UV-A for wavelength of 400 to 320 nanometers, mid UV or UV-B for wavelengths of 320 to 290 nanometers, and UV-C for wavelengths of 290 to 250 nanometers. Stratospheric ozone is very efficient at absorbing solar energy with wavelengths of less than 290 nanometers, but its efficiency drops off in the UV-B range (the range of wavelengths that give a sunburn) (Figure 6).

As stratospheric ozone is depleted, more energy in the UV-B range is allowed to pass though the stratosphere and troposphere and enter the biosphere - the sphere of plant, animal, marine, and soil organisms near the earth's surface (Figure 7).

Some evidence from Canada in Figure 8 shows UV radiation at the earth's surface increasing differently at different wavelengths, but the largest observed increase is at those wavelengths where ozone absorption is large.

Fortunately the increase is less in the summer, when humans have more skin exposed to the sun, than it is in the winter. The NOAA/EPA UV index provides a measure of skin damage caused by UV radiation based on weather forecasts.

Additional data from Montreal shows that the average daily flux of UV-B radiation is somewhat higher in 1993 than 1989 for both summer and winter (Figure 9). Updates on current ozone measurements from Antarctica can be obtained from the stations at Halley, Rothera and Vernadsky/Faraday. Canadian monthly and annual graphs of ozone levels demonstrate that ozone levels over several Canadian cities fall typically 5-10% below normal levels in the winter months. NASA also provides current maps of global ozone.

Regional average ozone decreases for other areas from 1960 to 1990 show weakly declining values over North America, Europe, and the Far East (Figure 10).

High latitudes generally are experiencing more rapid decreases, and decreases in winter are larger than in summer.

Ozone depletion is related to increases in skin cancer. A depletion of 2% total ozone is expected to lead to about one-half million additional cases of skin cancer and additional 9,300 deaths. Measured values of ozone in the latitude range of the United States currently are about 6-7% below the natural levels.

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