Abstract
Oxides of nitrogen (NOx) and volatile organic compounds (VOCs) released into the atmosphere can react in the presence of solar irradiation, leading to ozone formation in the troposphere. Historically, before clean air regulations were implemented to control NOx and VOCs, ozone concentrations were high enough to exert acute effects such as eye and nose irritation, respiratory disease emergencies, and lung function impairment. At or above current regulatory standards, day-to-day variations in ozone concentrations have been positively associated with asthma incidence and daily non-accidental mortality rate. Emerging evidence has shown that both short-term and long-term exposures to ozone, at concentrations below the current regulatory standards, were associated with increased mortality due to respiratory and cardiovascular diseases. The pathophysiology to support the epidemiologic associations between mortality and morbidity and ozone centers at the chemical and toxicological property of ozone as a strong oxidant, being able to induce oxidative damages to cells and the lining fluids of the airways, and immune-inflammatory responses within and beyond the lung. These new findings add substantially to the existing challenges in controlling ozone pollution. For example, in the United States in 2016, 90% of non-compliance to the national ambient air quality standards was due to ozone whereas only 10% was due to particulate matter and other regulated pollutants. Climate change, through creating atmospheric conditions favoring ozone formation, has been and will continue to increase ozone concentrations in many parts of world. Worldwide, ozone is responsible for several hundreds of thousands of premature deaths and tens of millions of asthma-related emergency room visits annually. To combat ozone pollution globally, more aggressive reductions in fossil fuel consumption are needed to cut NOx and VOCs as well as greenhouse gas emissions. Meanwhile, preventive and therapeutic strategies are needed to alleviate the detrimental effects of ozone especially in more susceptible individuals. Interventional trials in humans are needed to evaluate the efficacy of antioxidants and ozone-scavenging compounds that have shown promising results in animal studies.
Highlights
Reviewed by: Juerg Hamacher, Lindenhofspital, Switzerland Manoussa Ethel Fanny, Boston Children’s Hospital, Harvard Medical School, United States
volatile organic compounds (VOCs) or carbon monoxide (CO) participate in a series of complex photochemical reactions to produce free radicals that compete with ozone to react with nitric oxide (NO)
Because typically people spend the majority of time indoors, using outdoor concentration as a surrogate for ozone exposure would lead to greater overestimation of exposure for people living or working in more airtight buildings than for those living in less airtight buildings
Summary
The triplet oxygen (O3), is formed from the reaction between dioxygen (O2, the normal oxygen molecule) and a singlet oxygen (O, oxygen atom) in the presence of a third-body molecule able to absorb the heat of the reaction. In the absence of or at very low concentrations of VOCs or carbon monoxide (CO), ozone reaches a steady-state concentration depending on solar intensity, ambient temperature, and the ratio of NO2 concentration to NO concentration Under this condition, one NO2 molecule is converted via photolysis into one O3 molecule and one NO molecule; and ozone is, in turn, consumed by NO to regenerate a NO2 molecule. Another study predicted that changes in regional climate and globally enhanced ozone would increase ground-level ozone over most of the United States. Ozone during the fall reached the summer level in several Octobers in the 2000s and in 2010 over the southeastern United States This was attributed to enhanced emissions of biogenic isoprene (a VOC precursor of ozone) from water-stressed plants under a drying and warming condition [10]. This finding suggests that occurrences of a drying and warming fall in the future may lead to an extension of the ozone season from summer to fall in the regions with significant biogenic VOC emissions
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