What is ozone?
Stratospheric ozone is good. It occurs naturally in the upper atmosphere, forming a protective layer that shields the earth from the sun’s ultraviolet rays. This good ozone layer has been depleted by man made chemicals, leading to what people call the hole in the ozone.
Ozone is an unstable, reactive gas composed of three oxygen atoms. It is colorless to blue with a pungent scent described to be like chlorine bleach. Individuals vary in ability to smell ozone; some people can smell it at levels as low as 0.05 parts per million in a cubic foot of air. Ozone exists throughout the atmosphere, but has varied effects depending on where it’s found.
Ozone on the ground level is a harmful air pollutant, capable of damaging living cells like those present in the lining of human lungs. It’s also the main ingredient in smog.
Where does ground-level ozone come from?
Tropospheric, or ground level ozone, isn’t emitted directly into the air. Elevated concentrations are produced by reactions between sunlight and certain air pollutants. Specifically, nitrogen oxides (NOx) and volatile organic compounds (VOCs) accompanied by atmospheric oxygen.
Ozone typically reaches unhealthy levels on hot, sunny, calm-weather days in urban areas. It can still reach high levels during cold months with enough sun. Ozone can also be transported by wind, so more rural areas can also experience high concentrations.
How is ozone measured?
Chemical concentrations in the air are measured by units of the substance (milligrams, micrograms, nanograms, picograms, or other tiny units of measurement) per volume of air as cubic meters or cubic feet. Ozone is typically expressed as parts per million (ppm).
Instruments that monitor ground-level ozone use the principle of absorption of ultraviolet light. Ozone absorbs ultraviolet light at around 250 nanometers (nm), which is how the ozone layer in the stratosphere protects us from the UV radiation emitted from the sun. To determine ozone levels, the sensor pulls ambient air through a filter into an absorption chamber where UV light at a wavelength of 254nm shines through the sample.
The blue dot shows the sample path. The sensor detects a reduction in the intensity of UV light relative to the concentration of ozone in the sample.
The green dot shows the sensor measuring a “zero” sample, which is scrubbed of ozone using manganese dioxide. The zero concentration results in a higher light intensity reaching the detector.
The red dot shows the sensor measuring a third sample, irradiated by a UV light with a wavelength of 185nm to generate ozone.
These three readings combine to one accurate reading of ozone concentration. Calibration and referencing with laboratory standards ensures the instrument is responding correctly.
What are the standards for ozone in the United States?
National Ambient Air Quality Standards (NAAQS) for pollution specify a maximum amount of a pollutant to be present in outdoor air.
Currently, the EPA has primary (health-based) and secondary (welfare-based) standards in place for ozone. These standards are based on average numbers over the course of three years. Having a longer-term measurement period like that is a better indicator of overall air quality than data collected from just one year.
| Pollutant | Standard Type | Mass per Volume | Terms | Date Established |
| Ozone | Primary | 0.070 ppm | Not to exceed the 4th highest daily maximum 8-hour concentration, averaged across 3 years | 28 December 2015 |
| Ozone | Secondary | 0.070 ppm | Not to exceed the 4th highest daily maximum 8-hour concentration, averaged across 3 years | 28 December 2015 |
How does ozone affect people?
Ozone has a pronounced effect on health. It irritates and inflames the tissues lining human airways. Exposure to ozone can reduce the volume of air the lungs can take in, causing shortness of breath. In sufficient doses, it increases the permeability of lung cells, rendering them more susceptible to toxins and microorganisms.
Symptoms can include itching and watery eyes, sore throat, and swelling or congestion in nasal passages, as well as headaches, a heavy feeling in the chest, or fluid in the lungs. Higher levels of exposure cause more severe symptoms. Chronic exposure may lead to asthma.
Who is most at risk for adverse health effects from ozone exposure?
People with asthma, children, adolescents, and adults who work or exercise outdoors where ozone concentrations are highest, are at the greatest risk of harm from this pollutant. While there aren’t many studies on the effect of ozone on children, there are a number of reasons children are more susceptible to harm from airborne pollutants. Children and teens spend much more time outdoors and engaged in vigorous activities than adults. Children breathe more rapidly and inhale more pollution per pound of their body weight than adults. Also, children are less likely to notice their own symptoms and take action to avoid harmful exposures.
How does ozone affect the environment?
Ozone reduces the productivity of plants, causing damage in cells and destruction of leaf tissue. Therefore, the plants are less effective at photosynthesis and grow extra leaves to try and compensate, which results in less stored carbohydrates. These kinds of conditions make plants weak and susceptible to diseases, pests, cold, and drought.
Ozone reduces crop and timber yields, causing millions of dollars in economic losses. It disturbs the stability of ecosystems when more sensitive species die out. Ozone also reduces the production of roots, seeds, fruit, and leaves, reducing the amount of food available for wildlife.
This kind of pollution also can cause substantial damage to rubber, plastic, fabric, paint, and metal. Ozone exposure progressively damages the functional and aesthetic properties of products and shortens their lifespan, which means significant economic losses from maintenance, upkeep, and replacement of damaged products.