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Last Edited: 1/14/2015

Air Contaminants: Ozone

Ozone is a gas that is found far above us in the earth's atmosphere and in the air we breathe.  On this website, "ozone" generally refers to the ozone at ground-level, which is harmful to health and the environment.  To learn more, scroll down or select from a topic below.


What is ozone and how does it get in the air?

Ozone (O3) is a gas that you cannot see or smell.  It occurs naturally and as a result of human activity.

  • Stratospheric ozone ("good" ozone) occurs naturally in the sky about 10 to 30 miles above the earth's surface.  It forms a layer that protects life on earth from the sun's harmful rays.
  • Ground-level ozone ("bad" ozone) is formed from pollutants emitted from cars, power plants, and other sources.  These pollutants are called nitrogen oxides (NOx) and volatile organic compounds (VOCs).  When these pollutants come in contact with each other in the presence of heat and sunlight, they form ozone.  

Ground-level ozone is in the air we breathe. 

  • It is harmful to our health and to the environment
  • It causes lung inflammation, damages lung tissue, decreases the capacity of our lungs, and may cause asthma
  • Ozone is harmful to plants
    • It can damage plant cells and destroy leaf tissue
    • This could result in reduced crop yields and altered ecosystems

Ozone is found both in urban and rural areas.

  • Many urban areas, like big cities, tend to have higher levels of ozone
  • Rural areas have ozone, too, because wind can carry ozone (and the pollutants that form it) hundreds of miles away from their original sources
  • Ozone makes up most of what we call “smog"

Ground-level ozone is predicted to increase due to climate change.

  • With global warming, temperatures are predicted to increase and more heat will help convert nitrogen oxides and VOCs into ozone

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What are the health effects of ozone?

Breathing unhealthy levels of ozone can make people sick. 

  • It can make it easier to get a cold or pneumonia
  • It can cause coughing and sore throats, make heart and breathing problems worse, and damage the lungs
  • Repeated contact with ozone may permanently scar lung tissue and can cause permanent lung damage

Studies have shown that being exposed to ozone can:

  • Cause respiratory symptoms like coughing or pain when taking a deep breath
  • Cause wheezing and breathing difficulties during exercise or outdoor activities
  • Make asthma worse
  • Cause lungs to get inflamed
  • Temporarily decrease the lung capacity of healthy adults
  • Increase chances of getting respiratory illnesses like pneumonia and bronchitis

As a result of these studies, scientists now know that breathing in too much ozone can increase things like:

  • Asthma medication use
  • School absences
  • Hospital admissions and emergency room visits
  • Premature death from heart and lung disease

The California Air Resources Board (ARB) has quantified the health impacts of exposure to pollution from ozone and particulate matter in California.  You can also view ARB's ozone and health fact sheet for more information.

 

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Who is at risk?

When ozone levels are very high, everyone should be concerned about ozone exposure.   However, some people are more affected by ozone and can be sensitive to lower levels of ozone.  These people include:

  • People with asthma or lung disease
  • Children 
  • Older adults
  • Active people of all ages who exercise or work hard outside

People in these groups may feel the effects of ozone when they are outside, even for short periods of time or when doing light activities.  People in these groups should be careful to limit their outdoor activities during peak ozone hours. 

 

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How to reduce exposure

When ozone levels in the air are very high, everyone can reduce their exposure by:

  • Spending less time outdoors
  • Reducing physical activity levels (e.g. walking instead of running) 

People who are sensitive to ozone (e.g. those with lung disease) should be careful to limit their outdoor activity during peak ozone hours.  If windows are open, indoor ozone levels can be as high as outdoors.  Furthermore, some machines such as photocopiers, laser printers, and certain air purifiers can emit ozone indoors.

 

You can plan your daily activities to reduce exposure to ozone.

  • Know the peak times for ozone
    • Ozone levels are highest after noon and through early evening hours on hot, sunny days
    • Ozone season is usually May through October
  • Check the Air Quality Index (AQI) for your area
    • A list of regional air districts and links to current air quality data can be found here
    • Learn more about the Air Quality Index on the California Air Resources Board website

Local TV stations, radio programs, and newspapers report these air quality forecasts to tell you when ozone levels are likely to be unhealthy.


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Measures of Ozone

There are different ways to assess ozone in our environment.  For example, we can learn about short-term exposure by looking at how many days the ozone concentration exceeded the national standard in a year.  For long-term exposure, we can look at the average concentration of ozone for the year.  When looking at ozone, it is useful to choose a measure that will give us the information that we are interested in.

 

Below are the ozone measures currently available on our air quality data query.  To learn more about the measures, click below or scroll down:

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Number of days of daily maximum 8-hour average ozone concentrations over the CA standard (0.070 ppm)

Ozone concentrations are continuously measured and averaged over each 8-hour period.  For each county, we present the number of days that the ozone concentrations exceeded the California 8-hour standard, which is defined as the number of days in each year that the maximum 8-hour average ozone concentration was greater than 0.070 parts per million

What does this measure tell us? 

    • This measure is useful for assessing short-term exposures

    • This measure can tell us how many days per year county residents are exposed to unhealthy levels of ozone exceeding the federal standard

    • This measure can be used to communicate to sensitive populations (such as asthmatics) the number of days that they may be exposed to unhealthy levels of ozone

What can’t this measure tell us?

    • This measure cannot tell us what an individual living in any county is exposed to

    • This measure cannot tell us how ozone levels vary within a county

    • This measure cannot tell us what ozone levels populations in counties without monitors are exposed to

    • This measure does not provide information regarding the severity (maximum) concentration of exposures

See the latest results for this measure on our data query tool.  Learn about how ozone is measured and the limitations of air monitoring.


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Number of person-days for daily maximum 8-hour average ozone concentrations over the CA standards (0.070 ppm)

This measure is an estimate of the number of people exposed to high ozone concentrations in a county.  The measure is calculated by multiplying the number of days that monitored levels are above the CA standard in a given county and the population of that county.

What does this measure tell us? 

    • This measure is useful for assessing short-term exposures

    • This measure can tell us which counties have the most people impacted by ozone

    • This measure can tell us which counties are most in need of prevention and control strategies

What can’t this measure tell us?

    • This measure does not identify the number of potentially vulnerable populations in each county, such as the elderly and children, who are at greater risk for ill effects from air pollution

See the latest results for this measure on our data query tool.  Learn about how ozone is measured and the limitations of air monitoring.


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Number of days of daily maximum 8-hour average ozone concentrations over the U.S. standard (0.075 ppm)

Ozone concentrations are continuously measured and averaged over each 8-hour period.  For each county, we present the number of days that the ozone concentrations exceeded the U.S. 8-hour standard, which is defined as the number of days in each year that the maximum 8-hour average ozone concentration was greater than 0.075 parts per million

What does this measure tell us? 

    • This measure is useful for assessing short-term exposures

    • This measure can tell us how many days per year county residents are exposed to unhealthy levels of ozone exceeding the federal standard

    • This measure can be used to communicate to sensitive populations (such as asthmatics) the number of days that they may be exposed to unhealthy levels of ozone

What can’t this measure tell us?

    • This measure cannot tell us what an individual living in any county is exposed to

    • This measure cannot tell us how ozone levels vary within a county

    • This measure cannot tell us what ozone levels populations in counties without monitors are exposed to

    • This measure does not provide information regarding the severity (maximum) concentration of exposures

See the latest results for this measure on our data query tool.  Learn about how ozone is measured and the limitations of air monitoring.


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Number of person-days for daily maximum 8-hour average ozone concentrations over the U.S. standards (0.075 ppm)

This measure is an estimate of the number of people exposed to high ozone concentrations in a county.  The measure is calculated by multiplying the number of days that monitored levels are above the U.S. standard in a given county and the population of that county.

What does this measure tell us? 

    • This measure is useful for assessing short-term exposures

    • This measure can tell us which counties have the most people impacted by ozone

    • This measure can tell us which counties are most in need of prevention and control strategies

What can’t this measure tell us?

    • This measure does not identify the number of potentially vulnerable populations in each county, such as the elderly and children, who are at greater risk for ill effects from air pollution

See the latest results for this measure on our data query tool.  Learn about how ozone is measured and the limitations of air monitoring.


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