How dangerous is sunlight?



While most molecules of the element oxygen exist as oxygen gas, O2, some form molecules of ozone, O3. When O2 molecules absorb energy from electrical discharge and UV radiation some dissociate to individual O atoms. If these reactive oxygen atoms bind to a molecule of oxygen gas, they form ozone. If they bind to a molecule of ozone, they form produce two molecules of oxygen gas.
O + O2----------------O3
O + O3 -----------------> 2O2
Every day a cycle occurs in which 300 million tons of stratospheric ozone is created and just as much is destroyed leading to a steady state concentration. A typical O3 molecule lasts only a few minutes. There is only enough ozone in the atmosphere to form a layer around the earth about 1/8 inch thick. There is no such "ozone layer" per se since 90% of the ozone is found between 10 & 50 km above the earth mixed with other atmospheric gases.
Each molecule is unique in the wavelengths of electromagnetic energy that it absorbs. As previously mentioned, carbon dioxide absorbs infrared wavelengths (heat). Ozone absorbs ultraviolet light (UV light). This is fortunate since DNA also absorbs UV light. When UV light strikes DNA bases, it can cause two DNA bases (thymine bases, to be specific) to adhere to each other (forming a thymine dimer). When the cell tries to replicate its DNA, this dimer is confused for a single base and a mutation can result. There is so much UV light which reaches the earth from the sun that, were much of this energy not absorbed in the atmosphere, the organisms which live on land would suffer many mutations and life on land would be impossible. For the majority of life's history on earth, life on land was impossible-it was only after photosynthesis had produced enough atmospheric oxygen to create an ozone layer that life occurred anywhere other than the ocean.

thymine dimer

An ozone layer is necessary to protect terrestrial life from harmful UV rays. After a day in the sun, a typical epidermal cell has between 100,000 and 1 million damaged DNA regions as a result of UV light. Unfortunately, some molecules can destroy ozone without being changed in the reaction (characteristic of molecules known as catalysts) and thus one molecule of a catalyst can destroy many ozone molecules. There are several such catalysts including free radicals resulting from water and NO. Nuclear explosions inject NO into the stratosphere and thus can decrease ozone levels. Many feel that the nuclear testing that occurred during the 1960s had an adverse effect on the ozone layer and that a large scale nuclear war would drastically reduce the ozone layer.

Another catalyst of ozone degradation are molecules known as CFCs (chloroflourocarbons). These compounds (which contain chlorine, fluorine, and carbon; freon is the best known example) were invented in 1930 and, since they are inert, they made ideal replacements for ammonia and SO2 as refrigerants. They were later used as propellants in aerosol cans, components of plastic foams, solvents for oils, sterilizers for medical equipment, and other uses. Since these molecules are inert, they can reach the stratosphere (after about 5 years) unchanged. In the stratosphere, UV light reaches the CFCs and the carbon-chlorine molecules are broken, releasing chlorine atoms. Chlorine is a catalyst of ozone breakdown and 1 chlorine atom may destroy 100,000 ozone molecules before winds return it to the lower atmosphere where they form compounds that are ozone-safe (American Chemical Society, 1994).
While it was known in 1973 that CFCs can damage ozone (this finding was published in the scientific journal Nature), the industries relying on CFCs generated $8 billion and employed 200,000 people. Johnson Wax (the 5th largest CFC manufacturer) admirably stopped CFC production and Oregon became the first state to ban CFCs in aerosols. In 1976, the National Academy of Science agreed that CFCs damage ozone and consumer concern provided the impetus for ozone-safe products. By 1978 there was a ban on CFCs in aerosols. The reduction of CFC use was resisted by some and one former head of the EPA even dismissed the CFC-ozone connection as a "scare". In the early 1980s the government stalled the phase out of CFCs (which, once CFCs were classified as dangerous, was required by the Clean Air Act). In 1984 the EPA even sued the government for not complying with Clean Air Act regulations.

Concern over the ozone layer intensified in 10/84 when a "hole" in the ozone layer over Antarctica was detected by British scientists. NASA confirmed this finding the following year. During winter, permanent Antarctic clouds exist in which ice crystals serve as a surface for reactions involving CFCs and chlorine. The reactive forms of chlorine destroy ozone before they are dispersed. By 1987, ozone levels during winter had dropped to half the pre-1970 levels. Presently about 60% of the ozone is depleted from an area 3x the size of the continental U.S. More air movement occurs at the North Pole and the ozone loss, although serious, is less drastic than the situation observed over Antarctica. The southern ozone hole is expanding towards Australia and southern Chile. In 2002, the ozone hole over Antarctica elongated over time and split into two separate holes (Varotsos, 2004). The depletion of ozone over the Arctic can cause an ozone depletion of 30% during much of the year and 70% during spring (Tabazadeh, 2002). Severe ozone depletion in the spring of 2000 led to 2 weeks in which an area over the Arctic was essentially free of ozone (Bottenheim, 2002).
The observation of ozone holes eliminated much of the resistance to limiting CFC production and use. In 9/1987, the Montreal Protocol was reached in which 149 countries agreed to reduce CFC production and consumption. In 1988 a decrease in the amount of ozone over the Northern Hemisphere was observed (a reduction of 1.7-3.0%). In 1992, a 10-15% ozone loss measured over middle latitudes in Northern Hemisphere and 1993, a 15-25% depletion over same area. This was a serious loss-it is estimated that every 1% decrease in the ozone layer will lead to about a 2% increase in non-melanoma skin cancer incidence. These findings changed the policy of some CFC manufacturers which had been resisting CFC limitations. The seriousness of the risk was recognized and there was a 40% drop in CFC use from 1986 to 1991. The regulations put in place by the 1987 Montreal Protocol have decreased the production of ozone-depleting chemicals by more than 90% (99% of that from developed nations and 50% of that from developing nations) (Luken, 2005).
While great progress was made, not all problems were solved. Replacements for CFCs are sometimes (but not always) more expensive and some poorer countries have requested help in addressing the problem (which is commonly perceived to be a problem wrought by industrial nations). About a billion dollars were required for the Montreal Protocol's Multilateral Fund which pledged over two million dollars to the largest remaining CFC producers, China and India, to phase-out their production of these gases. Some of the replacements are capable of damaging ozone, although to a lesser degree, and are potent greenhouse gases. Finally, even though the response to this problem has been fairly rapid, CFCs and the chlorine they produce will be part of the atmosphere for decades to come. It is predicted that the concentration of ozone in the atmosphere will reach the pre-1980 level by the year 2040. Interestingly, some of this recovery is being promoted by greenhouse gases (Dyominov, 2005).


--after Kemp, 2004

The less ozone there is to absorb ultraviolet light, the greater the number of mutations and thus the greater the number of cases of skin cancer. The following two photos of me on a trip to southern Chile in 1991 while I was in the Peace Corps. I was near the city of Puerto Mont, about as far south in Chile as you could get by land without taking a boat headed towards Tierra del Fuego at the tip of South America. The ozone hole has expanded to the point where it affects the southern parts of South America and Australia. The first photo is of myself on a cold, windy beach and the second photo is of the following day with a bad sunburn.

me on beach sunburn

The increase in ultraviolet light exposure is blamed for increasing rates of skin cancer. From 1960 to 1986, skin cancer rates increased more than 2x among men and 3.5 times among women in the western U.S. (WHO, 1990a). In 1980, 1 in every 250 Americans contracted melanoma during their lifetime. Today the rate is 1 in 84 (Blatt, 2005).

The increased amounts of UV light can also increase the incidence cataracts and blindness, not only in humans, but also in animals. This has already been observed in southern South America. Ultraviolet light can suppress the immune system and result in a greater susceptibility to infectious diseases, cancer, and AIDS. UV radiation has an adverse effect on shrimp, crabs, zooplankton, and fish and it decreases plant and phytoplankton production. Phytoplankton is the ultimate food source for ocean animals and is important in the production of oxygen and removal of CO2. Decreases in phytoplankton of 6-12% have been measured under the Antarctica hole; corals and other invertebrates may suffer as well. Increased ultraviolet radiation is increasing plant damage in southern South America and Antarctica (Day, 1999; Rousseaux, 1999; Last, 1993).