Health effects

Air pollution

Red Alder leaf, showing the typical discolouration caused by ozone pollution.^[22]

There is a great deal of evidence to show that high concentrations of ozone, created by high concentrations of pollution and daylight UV rays at the Earth's surface, can harm lung function and irritate the respiratory system.^[15][23] A connection has also been known to exist between increased ozone caused by thunderstorms and hospital admissions of asthma sufferers.^[24] Air quality guidelines such as those from the World Health Organization are based on detailed studies of what levels can cause measurable health effects. Exposure to ozone and the pollutants that produce it has been linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems. According to scientists with the United States Environmental Protection Agency (EPA), susceptible people can be adversely effected by ozone levels as low as 40 ppb.^[25]

The Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, the EPA lowered its ozone standard from 80 ppb to 75 ppb. This proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 ppb, and the World Health Organization recommends 51 ppb. Many public health and environmental groups also supported the 60 ppb standard. On the other hand, the EPA had already designated over 300 mostly urban counties as out of compliance, and lowering the standard to 75 ppb put hundreds more in non-compliance. Lowering it further to 60 ppb would likely have left most of the US in non-compliance. Manufacturers, employers, and others argued that the cost of compliance with the lower standard would be prohibitive.^[25] The EPA has also developed an Air Quality Index to help explain air pollution levels to the general public. Eight-hour average ozone concentrations of 85 to 104 ppb are described as "Unhealthy for Sensitive Groups", 105 ppb to 124 ppb as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".^[26]

Ozone can also be present in indoor air pollution, partly as a result of electronic equipment such as photocopiers.

A common British folk myth dating back to the Victorian era holds that the smell of the sea is caused by ozone, and that this smell has "bracing" health benefits.^[27] Neither of these is true. The characteristic "smell of the sea" is not caused by ozone but by the presence of dimethyl sulfide generated by phytoplankton, and dimethyl sulfide, like ozone, is toxic in high concentrations.^[28]

Long-term exposure to ozone has been shown to increase risk of death from respiratory illness. A study of 450,000 people living in United States cities showed a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels such as Houston or Los Angeles had an over 30% increased risk of dying from lung disease.^[29][30]

Physiology

See also: trioxidane

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Ozone has been found to convert cholesterol in the blood stream to plaque (which causes hardening and narrowing of arteries). Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of the Department of Chemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.^[31]

When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed “Atheronals”, generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol^[32] as well as a secondary condensation product via aldolization.^[33]

Ozone has been implicated to have an adverse effect on plant growth, "...Ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus."^[34]

Safety regulations

Due to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Center for Occupation Safety and Health reports that:

"Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both by the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." ^[35]

To protect workers potentially exposed to ozone, OSHA has established a permissible exposure limit (PEL) of 0.1 ppm (29 CFR 1910.1000 table Z-1), calculated as an 8 hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 ppm.^[36] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.