by Daniel Weber, Ph.D.
Marine & Freshwater Biomedical Sciences Center, University of Wisconsin-Milwaukee
It’s summer and if you live in an urban region you have probably seen the color-coded ozone alerts, warning you of days when it is not safe to exercise or spend long periods of time outside. You may also have been confused, because although ozone at the ground level is bad for humans, ozone in the upper atmosphere is essential, and its depletion is a serious problem (you may have heard this referred to as the “hole in the ozone layer”).
The distinction between the “bad” ozone at ground level, e.g., cities, and the “good” and protective ozone that is being depleted in the upper atmosphere can be confusing. Each ozone type has unique effects on the health of humans and wildlife. This article will focus on ground level, or tropospheric ozone formation – the ozone responsible for the ozone alerts on the news.
Ozone can be very dangerous to human beings when we breathe it. It is not very water soluble, thus it penetrates deeply into our lungs whenever we inhale. Ozone’s high level of reactively damages cells and tissues. Ozone affects protein structure and function and leads to the formation of other chemical radicals that destroy the mucosal lining in the lungs, as well as the cellular structure. As cells and tissues become stressed due to ozone’s reactivity, they become more susceptible to bacterial infections. Another effect of O3 is the decrease in the lung immune system (pulmonary macrophages). In immunocompromised individuals, such as those on anti-cancer treatments or using drugs to prevent organ rejection after an organ transplant, ozone exposure could be lethal.
Ozone has also been shown to increase lesions in the alveoli of the lung, the small air sacs where the exchange of oxygen and carbon dioxide occur Thus, although ozone does not cause asthma or emphysema, it may increase the severity and frequency, especially in children with pulmonary diseases, of attacks if other confounding factors, such as pollen, dander, or other pollutants, are present. Tolerable levels of ozone are less than 1 part per billion (ppb) the concentration equivalent of one person in all of India. In urban areas, ozone levels are frequently in the 80-200 ppb range with peak levels as high as 350-500 ppb. That’s 100 times the background level and still a small amount, which makes our transportation choices that much more significant because too much ozone at ground level is a major health problem.
So what is ozone? To explain it in better detail, we have to look at some chemistry.
Ozone (O3) is composed of three oxygen atoms bonded together. The oxygen we breathe (O2) has two atoms and that extra oxygen atom makes all the difference in reactivity with biological tissues. Ozone is formed from reactions in the lower atmosphere from the photolysis of nitrous oxides, particularly NO2 — a component of automotive exhaust. NO2 is an inorganic molecule. Photolysis is the process by which light energy breaks chemical bonds, thus allowing additional reactions to occur. Chemically, this reaction is written:
O2 + light -> NO + O·
That monoatomic oxygen (O·) is critical for O3 formation because it is very reactive, especially with the oxygen we breathe (O2). So, in the presence of oxygen or nitrous oxides, ozone is formed. Because O3 is very reactive, the reverse reaction occurs just as fast:
O3 + NO -> NO2 + O2=
This is crucial because without the presence of hydrocarbons (produced by burning fossil fuels), ozone would not accumulate in cities. While this chemistry is critical to understanding ozone formation, it is the presence of hydrocarbons in the air produced by burning fossil fuels that lead to its accumulation (which is why we need to drive less and use bikes or mass transit more). Ozone reacts with light, especially at the short ultraviolet wavelengths (which is the key to understanding next issue’s stratospheric ozone discussion) to form O2 and that very reactive monoatomic O·. When O· reacts with the water vapor in the air it forms another very reactive chemical called a hydroxyl radical, or OH·.
When these hydroxyl radicals react with the hydrocarbons produced from vehicular exhaust they produce additional NO2 through other reactive compounds. By producing more hydrocarbons and therefore more NO2 as a direct result of our automotive activity we force the chemical balance toward more O3 (i.e. the reverse reaction of O3 reverting to O2 and NO2 cannot keep up with the excess NO2 being produced).
Ozone becomes problematic by accumulating rather than being carried away by wind currents. This circumstance occurs during periods of temperature inversions that trap ozone at ground level. Normally, temperature decreases with increasing altitude, but on particularly hot days, a layer of air can be produced that warms with increasing altitude. Because warm air is lighter than cold air, it rises until the naturally colder air of higher altitude traps it. Inversion layers create barriers to ozone dispersion so that it accumulates to levels that endanger human health.
There are times and places, however, where the presence of ozone is critically necessary. It is this “good” ozone (over 90% of the Earth’s total ozone) that occurs in the stratosphere, a region 8-17 kilometers above the planet’s surface. This is the ozone typically referred to in regard to the “hole in the ozone layer.”
Ozone can actually have protective effects as long as there is a sufficient supply of it. That’s because it absorbs ultraviolet (UV) radiation from the sun, which breaks apart the bonds that hold oxygen atoms together that form breathable oxygen (O2) and ozone (O3). Because breaking bonds requires energy, this process absorbs the intense, dangerous energy in UV radiation. This is important because UV radiation can cause many disruptions in natural systems, affecting human health, the well-being of wildlife, and nutrient cycling in terrestrial and aquatic ecosystems. Having too little ozone in the ozone layer (the “hole in the ozone layer”) makes humans and the environment vulnerable to UV radiation.
Today’s “hole in the ozone layer” was caused by the use of CFC’s in the 20th century. (For the chemistry of the depletion of the ozone layer, see below.) The preponderance of ozone-destroying chemicals in the upper atmosphere has lead to significant reductions in the thickness of the ozone layer. For example: a 10 million square mile region over the Antarctic (that is 25% larger than the surface area of North America) developed in the winter of 1998. Since 1956, total ozone over Antarctica has decreased by half and correlates with the increased use of CFCs during that time period, holes over the Arctic Ocean were discovered during the winter of 1999-2000 and 60% of the ozone at 60,000 feet above the Arctic was lost. Even though CFCs generally are banned worldwide, their longevity has meant that their effects are still evident.
The main effect of reduced stratospheric ozone is an increase of UV radiation on earth. This has impacts on human health and the environment. While UV radiation is beneficial for producing vitamin D in our bodies, it can age the skin and damage DNA (our cell’s genetic code). DNA mutations can cause certain forms of skin cancer (e.g., melanoma) or suppression of our immune system, which may lead to non-Hodgkin’s lymphoma. It is estimated that increasing levels of UV radiation may increase the incidence of cataracts, as well as cornea, lens, and retina damage in the eye.
While animal life is subject to similar dangers as humans (e.g., skin cancers), plant life has a whole suite of different reactions to UV. These changes may include increased susceptibility to pathogens, decreased palatability to herbivores, and altered life-cycle events such as flowering. Animals who depend upon predictable plant patterns may be adversely affected if these patterns change. Because these particular changes in growth and physiology are often species specific, it has been hypothesized that changes in the intensity of UV radiation may alter the interactions within a particular habitat and, therefore, ecosystem dynamics.
Affects on aquatic systems are often limited to the surface (down to 15 m from the surface) due to the ability of water molecules to absorb UV radiation. In aquatic, as in terrestrial, ecosystems animal larvae, e.g., amphibia and sea urchins, are particularly sensitive. Corals, as well as their symbiotic algae from which corals derive much of their food, are adversely and directly affected by UV radiation.