With all the worries about the vanishing ozone layer, concerns about the rising level of ozone may seem odd. Just last year, the US Environmental Protection Agency (EPA) celebrated the 40th anniversary of the Clean Air Act, a major component of which is focused on ozone protection. Two years prior, in 2008, it lowered the ozone standard from 0.08 parts per million (ppm) to 0.075 ppm. How can we want to both preserve and abate ozone at the same time?
“Good Up High, Bad Nearby”
Both the good and bad “types” of ozone actually have the same chemical composition (O3), but the major difference between the two is location. The ozone layer is in the upper stratospheric layer of the atmosphere, where it plays the protective role the media often portrays. On the other hand, there is ground-level ozone, located in the troposphere, the lower layer that contains what humans breathe in daily. Besides the location difference between stratospheric and ground-level ozone, the effects are quite dissimilar as well. A helpful mnemonic developed by the EPA is “Good Up High, Bad Nearby.”
Due to the positional difference, the way ozone is created in both layers is not the same. In the stratosphere, ozone comes from UV rays breaking the bonds of diatomic oxygen (O2), creating two free oxygen atoms that can quickly react with other O2 molecules to form O3. Ultraviolet rays can also split ozone into O2 and a free oxygen atom, but these can react with each other to reform ozone. These two reactions are exothermic and protect the Earth from harmful UV effects by converting the radiation into heat. Normally, this cycle maintains a constant level of ozone, but chemicals such as chlorofluorocarbons (CFCs) catalyze the breakdown reaction, shifting the equilibrium toward ozone layer depletion.
In the troposphere however, most of the ozone is generated in a different manner. Ozone is not directly emitted but rather created from two main categories of precursors – nitrogen oxides and volatile organic compounds (VOCs) – that react in the presence of sunlight. The components cannot simply be eliminated as they stem from multiple anthropogenic and natural sources. Although industry and transportation are common culprits, trees and vegetation are also important sources. While urban centers have higher ozone levels as expected, in some major agricultural communities, natural ozone precursors are actually more of a problem than man-made ones.
How Ozone Impacts Health
Exposure to ozone has now been firmly linked to respiratory symptoms (e.g. coughing, increased asthma attacks, difficulty breathing, lung inflammation) and consequent hospital admissions. Several years ago, however, the evidence had been unclear regarding the impact of ozone on human mortality because different studies yielded conflicting results. Professor Michelle Bell, Professor of Environmental Health at the Yale School of Forestry & Environmental Studies, and her colleagues hypothesized that this could be attributed to factors such as datasets of insufficient size and inadequate control of possible contributing factors such as other pollutants or weather. For example, higher ozone levels are present on hotter days when more sunlight increases the rate of ozone synthesis, so seasonal control is needed.
Because epidemiological studies cannot be strictly controlled like experiments in a lab, the resulting real world data are naturally messy albeit very compelling if a trend can be found. These convoluting factors can be accounted for with statistical models and sensitivity analysis, which help to confirm that the discovered relationship is actually due to the variable under investigation. Bell, though, emphasizes that this research is not completely math-based. “I think that the best work of this type comes from people who have taken time to really understand the underlying systems, how ozone formation is affected by weather and the atmospheric systems,” she says. “I do not view this at all as just a mathematical exercise.”
Accordingly, Bell does her best to avoid the above possible pitfalls. Her 2004 large-scale study spanning 95 U.S. cities (approximately 40% of the American population) showed that short-term exposure over just a few days to elevated ozone levels still led to premature mortality. She found that a 10 part per billion (ppb) increase in the previous week’s ozone concentration was associated with a 0.52% increase in daily mortality, even after taking temperature from heat waves into account. In a follow-up project in 2007, ozone health effects are not simply artifacts of harmful particulate matter effects, as emission sources often produce ozone precursors as well.
Although many people stop coughing after short-term exposure to heightened ozone levels ceases, both short-term and long-term exposures may have longer-lasting, detrimental effects. Four main groups of people are more susceptible to these effects: people with respiratory illnesses, which can be aggravated by ozone; adults active in the outdoors, as ozone intake increases with deeper and faster breathing; children because they often engage in outdoor summer activities and breathe more air per body weight than adults do; and healthy people who are somehow naturally more sensitive to ozone. However, anyone can be affected. Having no symptoms does not mean that ozone is not having an effect on the body. People who appear to have developed resistance after living in areas with high ozone levels can continue to incur lung damage.
In fact, another one of Bell’s studies in 2006 concluded that there was no such thing as a threshold level of ozone below which the chemical had absolutely no health impact. Even natural background concentrations of this chemical are related to increased mortality.
Translation to Policy
If no perfectly safe ozone level exists, what type of policy should we pursue? Because ozone precursors come from a wide variety of sources, even some natural, the solution is not as simple as targeting one specific pollutant. However, current environmental policies already often overlap and seem to be headed in the right direction. This issue of co-benefits emerges when comparing policies to curb greenhouse gas emission with policies that aim to improve air quality. Although people think of them separately, they should be considered together since the suggested actions often achieve both goals and improve human health in the short term.
To further research health impacts of ozone and air quality in general, the scientific community has recently been shifting from single pollutant studies to those that consider complex combinations of pollutants, as the body takes in all pollutants simultaneously. Research can possibly identify particular groups or chemical forms of particles that are the most harmful. So far, though, Bell’s studies and those conducted by other scientists have already been used by organizations, such as the EPA and WHO, to create regulations for ozone levels. Bell states that the purpose of doing her kind of research is not to dictate public policy but to help policy makers make informed decisions.
About the Author
NANCY HUYNH is a junior Molecular, Cellular, and Developmental Biology major in Silliman College. She used to be confused by why policy makers would possibly want to abate ozone.
The author would like to thank Professor Michelle Bell for her time, explanation of the difference between tropospheric and stratospheric ozone, and continued work on environmental health.
Bell, Michelle L., Richard Goldberg, Christian Hogrefe, Patrick L. Kinney, Kim Knowlton, Barry Lynn, Joyce Rosenthal, Cynthia Rosenzweig, and Jonathan A. Patz. “Climate change, ambient ozone, and health in 50 US cities.” Climatic Change 82 (2007): 61-76. doi: 10.1007/s10584-006-9166-7.