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Understanding Ozone

 "We All Share the Air" video courtesy of ADEQ.


Ozone (O3) is a problem pollutant in PIma County during ozone season which runs from April through September.  Pima County Department of Environmental Quality monitors ozone at eight different monitoring sites in eastern Pima County. The precursor pollutants that combine to form ground-level ozone come from many different sources, including automobiles, industries, power plants and even certain types of vegetation. Although weather plays a role in how ozone is created, as individuals we can take actions on a daily basis to help keep the air healthy to breathe.

Where do we stand with the EPA standard?

The United States Environmental Protection Agency (EPA), has classified ozone as a criteria pollutant because of the health problems it can cause. EPA has established National Ambient Air Quality Standards (NAAQS) of 0.070 ppm averaged over 8 hours. To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.070 ppm (effective October 2015).

During the summer of 2018, Pima County DEQ's monitors indicated that the ozone standard was exceeded enough times to be considered in violation of the EPA standard. The current EPA designation for Pima County as "attainment" of the ozone standard will remain in effect until EPA takes further action.

A 2018 Ozone Status Report by Pima Association of Governments provides additional information regarding ozone in Pima County, as well as control measures that could be implemented in the future.
How is ozone formed?

What is ozone?

Ozone is a colorless, pungent, highly reactive gas, considered a secondary pollutant (it is not emitted into the air directly).

Most oxygen in the air is O2 - two joined oxygen atoms. O2is the oxygen that sustains life. Ozone is O3 - three oxygen atoms joined together. Ozone is ready to react with whatever it meets. This makes it very useful for cleaning and disinfecting, but when it comes in contact with living tissues like our lungs it can cause damage and illness. Ozone can also corrode building materials, statues and monuments, and natural rock features in the landscape.

Ground-level ozone is the principal component of what some regions call "smog" and is caused primarily by mobile source emissions (motorized vehicles) in Pima County. Other sources of emissions that form ozone include industrial, power plant, and biogenic emissions from certain types of vegetation. 

How is ozone pollution different than the ozone layer?

Distinct from the stratospheric ozone layer, which lies 10 km above the earth’s surface, the air pollutant ozone is in the troposphere and is formed by photochemical reaction driven by the action of intense ultraviolet light on the precursor pollutants oxides of nitrogen (NOx), and volatile organic compounds (VOCs). During ozone season (April through September) ozone concentrations in Pima County rise in the late morning, peak in the afternoon, and decrease at night. The highest levels of ozone are dependent on overall weather conditions and occur most frequently on sunny days when the air is stagnant.

The ozone layer in the upper stratosphere protects life on earth from sun's harmful ultraviolet rays and is far above the air that we breathe. This protective shield becomes weaker when certain gases are able to travel to the ozone layer and destroy the ozone molecules. These destructive gases come from products like refrigerators, air conditioners, cleansers, and aerosol cans. The gases are called halons and chlorofluorocarbons (CFCs).

What can we do to reduce ozone?

There are numerous ways we can reduce the pollutants that form ozone including driving and idling our vehicles less; sharing rides, taking transit, biking and walking more; conserving electricity; refueling our vehicle in the evening; keeping our tires properly inflated; and avoiding the use of lighter fluid and gas-powered lawn and garden equipment.

Certain types of trees emit one of the essential ingredients for ground-level ozone, volatile organic compounds (VOC). Learn more about which trees have low VOC emissions, which are the best trees to plant in Southern Arizona, and the many benefits to increasing the tree canopy in the Tucson area.

Things that affect ozone levels

The amount of ingredient gases

More ingredient gases (VOC's and NO2) can mean more ozone. Towns and cities that have more traffic or more industrial plants have a higher potential for ozone formation, especially towns that also experience many warm sunny days with little wind.

Weather and seasons: warm sunny days mean more ozone.

Since sunlight is the engine that drives ozone formation, warm sunny days should have more ozone than cool or cloudy days. Wind can also play a role. On windy days the wind can disperse the ozone, causing levels to drop. Ozone pollution can be especially bad during summer heat waves when the air does not mix very well and air pollution doesn't disperse.

Time of day: ozone levels build during the day.

On a clear day, ozone levels can continue to rise all day long, and then decrease rapidly after sunset. Since sunlight and the ingredient gasses each usually increase during the day, ozone formation also increases. When the sun goes down, there is no energy for ozone formation and fewer ingredient gases - so ozone levels drop. 

Ozone Health Effects

The properties that make ozone a powerful cleaner, disinfectant, and bleaching agent also make ozone dangerous to living tissues. When it comes in contact with living tissues, like our lungs, ozone attacks and damages cells lining the airways, this causes swelling and inflammation. Some have compared ozone's effect to a sunburn ... inside your lungs.

Other health effects include:

  • Irritation of the airway: a cough, an irritated throat, or an uncomfortable feeling in your chest.
  • Reduced lung function: you may not be able to breath as deeply or vigorously as you normally would.
  • Worsened Asthma: ozone can aggravate the effects of asthma (see Asthma below).
  • Potential health effects: ozone may aggravate the effects of emphysema and bronchitis, and may reduce the body's ability to fight infections in the respiratory system.

Who should watch out for ozone?

High ozone levels can affect anyone, but some groups of people are particularly sensitive to ozone.
  • Children: They spend more time outdoors, are more active, and their airways are not fully developed.
  • Adults exercising outdoors: Healthy persons engaged in physical activity breathe faster and more deeply. This increases the amount of ozone flowing into the lungs.
  • People with respiratory disease: Ozone can further irritate the airways of persons who already have diseases of the lung or airways. Ozone can irritate the already sensitive airway of someone with asthma. When ozone levels are high, more asthmatics have asthma attacks that require a doctor's attention or the use of additional medication. One reason this happens is that ozone makes people more sensitive to allergens, which are the most common triggers for asthma attacks. (Allergens come from dust mites, cockroaches, pets, fungus, and pollen.) Also, asthmatics are more severely affected by the reduced lung function and irritation to the respiratory system caused by ozone.
The degree of adverse respiratory effects produced by ozone depends on several factors, including concentration and duration of exposure, climate characteristics, individual sensitivity, preexistent respiratory disease, and socioeconomic status (1,15, 16, 17).

Both the level of physical activity and the sensitivity of the individual are factors in determining the adverse health effects of ozone. 

Another factor that increases adverse effects of ozone is socioeconomic status. People with low incomes are less likely to have air conditioners in their homes and thus are more likely to keep their windows open during the summer months when ozone levels are highest. Differences in areas of residence, also related to socioeconomic status, may affect the likelihood of being exposed to peak concentrations of certain air pollutants (1, 19)

Weather also plays an important role in the relationship between ozone pollution and health. Meteorological conditions influence the chemical and physical processes involved in formation of ozone. In a study conducted in Belgium during the summer, outdoor temperatures combined with high ozone concentrations were assumed to be likely causes of the important excess mortality (9). In another study conducted in New Jersey a strong relationship between summertime ozone concentration and emergency department visits for asthma was observed (8).

Two of the most important factors are the concentration of ozone and duration of exposure. Numerous epidemiological studies show the relationship between health effects and specific ozone ranges. EPA has gathered information about health effects through research, studies comparing health statistics and ozone levels in the communities, and controlled testing of human volunteers.

The EPA has developed the Air Quality Index (AQI) for reporting the levels of ozone and other pollutants, and their effects on human health. The AQI scale has been divided in different categories, which range from 0 to 500. Each category corresponds to a different health impact (Table1). The NAAQS for ozone is 0.070 ppm averaged over 8 hours.


Several pathophysiologic pulmonary processes result from ozone exposure. As a potent oxidant, ozone is extremely irritating to the respiratory system. It is capable of reacting with a variety of extracellular and intracellular biomolecules, and produces disruptive changes that may be measured by alterations in pulmonary function. In addition, ozone is less soluble than other irritant gases. It can penetrate more effectively through the tracheobronchial tree to the pulmonary regions of the respiratory system, inducing injury in resident lung cells, and causing an influx of inflammatory cells (5,15). Exposures lower than 0.08 ppm are sufficient to induce alterations of enzyme activity and to initiate an inflammatory reaction in the lung, inducing significant increases in neutrophils, protein, prostaglandin E2, interleukin-6, lactate dehydrogenase, and antitrypsin (5). These enzymes and mediators are usually associated with cell edema and at sufficient ozone concentrations, cell death.

Air Quality Index

8-Hour Average Ozone Concentration (ppm)  Air Quality Index Values  Air Quality Descriptor  Health Effects
 0.0 to 0.054  0 to 50  Good No health effects are expected.
 0.055 to 0.070  51 to 100  Moderate Unusually sensitive individuals may experience respiratory effects from prolonged outdoor exertion if you are unusually sensitive to ozone.
 0.071 to 0.085  101 to 150 Unhealthy for Sensitive Groups Member of sensitive group may experience respiratory symptoms (coughing, pains when taking a deep breath).
 0.086 to 0.105  151 to 200  Unhealthy Member of sensitive group have higher chance of experiencing respiratory symptoms (aggravated cough or pain), and reduces lung function.
 0.106  to 0.200   201 to 300  Very Unhealthy Members of sensitive groups experience increasingly severe respiratory symptoms and impaired breathing.

For information regarding updated studies of ozone and health, please refer to U.S. EPA website.

0.106 (8-h) TO 0.200 ppm (VERY UNHEALTHY)

At ozone concentrations from 0.106 to 0.200 ppm, sensitive people experience severe respiratory symptoms and impaired breathing.

Recent studies of humans exposed to these ozone concentrations have shown pulmonary function impairment during heavy exercise (7). Another study, conducted in Mexico City, shows that exposures from 0.170 to 0.250 ppm 1-h, increase the occurrence of respiratory symptoms, such as cough, phlegm, difficulty in breathing, and reduce PEFRs among children with mild asthma (2). In addition, ozone exposure to 0.30 ppm 1-h induces lower airway inflammation. This is manifested by PMN influx measured by bronchoalveolar lavage (3). Also, at this concentration with continuous exercise, FEV1 decreases.

0.086 to 0.105 ppm (UNHEALTHY)

8 HOURS EXPOSURE. Reduction in lung function is observed with exposures of <0.12 ppm over 6-8 hours with moderate exercise, manifested by decrements in FEV1. (4,5).


8 HOUR EXPOSURE. In accordance with AQI, sensitive people, active children and adults, and people with respiratory disease under prolonged outdoor exertion, may experience respiratory symptoms such as coughing or pain when taking a deep breath, and reduced lung function, which can cause some breathing discomfort.

A series of studies conducted in the USA (1,3,6) demonstrated that with 0.09 ppm ozone, the number of hospital visits for asthma increased and people undergoing moderate exercise increased their sensitivity to ozone. Also, 0.100 ppm ozone induces neutrophilic influx into the airway and resulting inflammation, and a decrease in forced expiratory volume (FEV1) and PEFR in asthmatic people (children and adults).

0.055 to 0.070 ppm (MODERATE)

8 HOURS EXPOSURE. Sensitive people may experience respiratory effects from prolonged exposure to ozone during outdoor exertion. In addition, other studies (1,6,8) have demonstrated that ozone concentrations at 0.080 ppm produce adverse effects on human health such as PEFR decrements in asthmatic children (6), decrements in FEV1 with intermittent exercise in healthy men (5,7), and increased hospital visits for asthma (1,8).

to 0.054 ppm (GOOD)

In accordance with NAAQS and AQI, at ozone levels from 0.0 to 0.54 no health effects are expected and the air quality is considered "GOOD". However, recent studies have demonstrated that at these concentrations ozone can exert adverse health effects.

1 HOUR EXPOSURE. A study conducted in Brisbane, Australia by Simpson et al. demonstrated an association between 0.030 ppm ozone and daily mortality in the elderly (11). In a study in Mexico, the relationship between ozone exposure in asthmatic children (5-13 years of age) and mild asthma was evaluated. Exposure to 0.050 ppm increased the occurrence of lower respiratory symptoms such as cough, phlegm and difficulty breathing, and reduced PEFRs. A different study suggests that 0.065 ppm ozone increases respiratory symptoms in asthmatic children. (5).

8 HOURS EXPOSURE. No information available.

24 HOURS EXPOSURE. Sartor and co-authors (1994) analyzed low levels of ozone and daily mortality in Belgium. This study demonstrated a relationship between 0.050 ppm ozone, high temperatures, and the number of daily deaths (9). An increase in elderly deaths with 0.034 ppm for 24-h was also observed (9, 11). Schwartz utilized Medicare records for the years 1986-1989 to study the association between ozone concentrations and respiratory admissions among elderly. In this study a significant relationship was observed between 0.050 ppm 24-h ozone concentration and hospital admission for pneumonia. (12).

Ground-Level Ozone in Pima County

Pima County Department of Environmental Quality (PDEQ) monitors ground-level ozone at eight different monitoring stations throughout eastern Pima County in compliance with the U.S. Environmental Protection Agency (EPA) requirements. The Clean Air Act requires EPA to review the National Ambient Air Quality Standards (NAAQS) periodically to see if they need to be modified.  In 2015, after reviewing numerous scientific and health studies, EPA changed the gound-level ozone standard to be more protective of public health. 

Up until the summer of 2018, PIma County had not violated EPA's health standard for ozone. In April of 2018, Pima County was designated as in attainment of the ozone standard by the EPA and that designation will remain in effect until EPA talkes further action.

Preliminary data from the PDEQ indicates that the air quality monitor at Saguaro National Park East violated the EPA ozone standard in August of 2018. The data will need to be validated and certified by EPA before it becomes official in March of 2019. Once the data is certified there will be a period of time before any decisions are made regaring the next steps for this region. 

In the meantime, we can take actions on a daily basis that will reduce the emissions that contribute to the formation of ozone and protect the health of those at risk, by driving our vehicles less, reducing engine idling, conserving energy and planting low VOC-emitting trees


Serious respiratory tract responses are induced by ozone, such as reduction in lung function, aggravation of preexisting respiratory disease (such as asthma), increases in daily hospital admission and emergency department visits for respiratory causes, and excess mortality. The adverse effects produced by ozone on the respiratory system depend on factors such as individual sensitivity (children, healthy adults doing outdoor exercise, people with preexistent respiratory disease and elderly), socioeconomic status, climate characteristics, and concentration and duration of exposure (1,15, 16, 17).

EPA has established National Ambient Air Quality Standards (NAAQS) of 0.070 ppm averaged over 8 hours. To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.070 ppm (effective October 2015). The Air Quality Index (AQI) reports the levels of ozone and other pollutants, and their effects on human health. According to the AQI, NAAQS and recent epidemiological studies, ozone concentration at 0.070 ppm and above for an 8-hour exposure, can in sensitive groups produce adverse health effects including lung function decrements, aggravation of preexisting respiratory disease, increases in daily hospital admissions and premature mortality. Those people may experience respiratory symptoms such as coughing, pain when taking a deep breath, and reduction in lung function, which can cause breathing discomfort. These symptoms worsen when ozone concentration increases. Other less severe effects such as decreased lung function and diminished athletic performance have been observed in this ozone range in healthy individuals.

According to the AQI, adverse health effects are not expected at 0.0 to 0.059 1-hour and 8-hour ozone exposure. However, epidemiological studies have provided information that adverse effect of ozone can be observed with exposure to low ozone concentrations over 1-hour, 8-hours and 24-hours (2,4,9,10,11,12,13). Sensitive groups are more influenced by these adverse effects. In addition, a number of studies have shown that the existence of other pollutants and weather conditions may worsen the adverse health effects seen with low-level ozone exposure.


White, M.C., et al. Exacerbations of childhood asthma and ozone pollution in Atlanta. Environmental Research. 65: 56-58, 1994.

Romieu, I., et al. Effects of intermittent ozone exposure on peak expiratory flow and respiratory symptoms among asthmatic children in Mexico City. Archives of environmental health 52:5 Sep/Oct 1997. 368-375.

Frisher, T.M. et al. Ambient ozone causes upper airways inflammation in children. American Review of Respiratory Disease. 148: 961-964, 1993.

Gielen, M.H., et al. Acute effects of summer air pollution on respiratory health of asthmatic children. American Journal of Respiratory and Critical Care Medicine. 155: 2105-2108, 1997.

American Journal of Respiratory and Critical Care Medicine. 153: 4-50, 1996.

Krzyanowski, M., et al. Relation of peak expiratory flow rates and symptoms to ambient ozone. Archives of Environmental Health. 47: 107-115, 1992.

Mckittrick, T., et al. Pulmonary function response to equivalent doses of ozone consequent to intermittent and continuous exercise. Archives of Environmental Health. 50:2 153-158, 1995.

Cody, R.P., et al. The effects of ozone associated with summertime photochemical smog on the frequency of asthma visits to hospital emergency departments. Environmental Research 58, 184-194, 1992.

Sartor, F., et al. Temperature, ambient ozone levels, and mortality during summer, 1994, in Belgium. Environmental Research. 70: 105-113, 1995.

Simpson, R.W., et al. Association between outdoor air pollution and daily mortality in Brisbane, Australia. Archives of Environmental Health. 52:6 Nov/Dec 442-454, 1997.

Gerard, H. et al. Effects of ambient particulate matter and ozone on daily mortality in Rotterdam, the Netherlands. Archives of Environmental Health. 52: 6 455-463, 1997.

Schwartz, J. PM10, ozone and hospital admissions for the elderly in Minneapolis-St. Paul, Minnesota. 49:5 366-374, 1994. (environmental Protection Agency).

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Maria A. Fierro, M.D., Mary Kay O'Rourke, Ph.D., and Jefferey L. Burgess, M.D., M.P.H.

The University of Arizona, College of Public Health