Ozone garden outside of the NSF NCAR Mesa Lab
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Did you know that ground-level ozone is a serious pollution problem? Ground-level (“bad”) ozone is toxic to plants and animals – including humans. It is different from the ozone layer (“good” ozone) high in the atmosphere that blocks the Sun’s harmful ultraviolet (UV) rays, even though it is the same chemical in both places. Ground-level ozone is harmful to us because it is in the air we breathe.

Although ozone is invisible, its effects can be observed on the leaves of certain plants. Ozone sensitive plants develop symptoms on their leaves that we can see, telling us when high levels of ozone are present in the air around us. Because they provide this information, they are called bioindicator plants. However, there are still lots of questions when it comes to ozone and plants:

  • What ozone concentrations cause damage to plant leaves?
  • How severe does the damage get?
  • Is it the same everywhere?

Scientists are trying to understand the answers to these questions. Will you help us by collecting data? Your observations of ozone damage will help answer these questions.

Ozone is a gas made up of three oxygen molecules (O3) that is found in two layers of the atmosphere that surrounds the Earth: The stratosphere (“good” ozone) and the troposphere (ground-level or “bad” ozone). Ozone in the stratosphere, 12-19 miles above the ground, forms naturally and shields the Earth from harmful solar radiation. Ozone that forms at ground-level is an air pollutant and is dangerous to breathe.

Good and Bad Ozone Diagram

Unlike some sources of air pollution, ground-level ozone isn’t emitted directly from a source such as tailpipes or smokestacks. Ground-level ozone forms when two types of pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs), react in sunshine. The highest ozone levels are typically found in the afternoon on hot, calm, sunny days, although ozone can be found year round.

The Source and Creation of Ozone

Ozone enters plant leaves through tiny pores on the underside of the leaves (stomata). Keeping stomata open is necessary so that the plants can get carbon dioxide from the air, which they turn into sugars for food during a process called photosynthesis. At the same time, however, ozone also gets inside the leaf and damages parts of the leaf cells that make the sugars. This can ultimately reduce the growth of the plant, reduce the production of wood and fruits and vegetables in timber and crop plants, and decrease the amount of carbon stored in plant tissues.

Plants are able to protect themselves from ozone damage in several ways. For example, antioxidants, like vitamin C, can protect against ozone damage, so plants with more antioxidants are less susceptible to ozone damage. Also, plants protect themselves by closing their stomata to reduce the amount of ozone getting inside their leaves. Plants close their stomata for other reasons, like when they are stressed by drought, and this also helps to protect the plant against ozone damage. Plants that have smaller and fewer stomata are typically less susceptible to ozone damage because less ozone enters the leaf.

Cross Section of a Leaf

Ground-level ozone causes muscles in the lungs to contract, narrowing pathways that carry air into and out of the lungs and making breathing difficult. Exposure to high ozone levels can cause a sore throat, coughing, lung inflammation, and permanent lung damage. Ozone decreases lung capacity by anywhere from 15 to more than 20 percent in sensitive individuals, and is particularly harmful to children, elderly, and people with respiratory illnesses. Similarly, animals (pets, livestock, wildlife) can also experience reduced lung functioning.

The Effects of Ozone on Human Lungs

Ground-level ozone pollution is also hazardous to plants, damaging plant cells and destroying leaf tissues. This reduces the plant’s ability to produce its own food through photosynthesis, decreasing the amount of food that agricultural crops produce, the amount of wood that trees produce, and the amount of carbon dioxide that plants remove from the atmosphere. It also weakens the plants and can make them more susceptible to disease, pests, cold, and drought. One study found that nine billion dollars of U.S. corn and soybean crops were lost each year to ozone between 1980 and 2011.

Ozone damage on sensitive snap bean plants.

Ozone Damage on Sensitive Snap Bean Plants

We need to cut down on our use of fossil fuels, reduce the amounts of VOCs emitted, and generally use less energy. This includes:

  • Making smart transportation choices: Choose a low-pollution vehicle, drive less (combine trips, carpool, work from home, bike, walk, or take public transportation), keep your car properly tuned, don’t idle for longer than 30 seconds, refuel in the evening, and stop fueling at the first click.
  • Taking action at home: choose renewable energy, choose low VOC products, use electric powered lawn and garden tools, mow in the evening, and forgo the fire on days with poor air quality.
What you can do to keep the air safe

Since we know that ozone is bad for human health and our ecosystems, scientists are working to better understand where ozone sources (NOx and VOCs) are coming from and how they move through the atmosphere. This will help us understand how to lower ozone levels to keep human and ecosystem health safe.

Until recently, ozone measurements have been limited to stationary monitors. For example, the Environmental Protection Agency has set up monitoring stations across the U.S. that are capable of tracking the variation in ozone levels for the surrounding areas. However, there are still large areas within the U.S. and throughout the world without ground-level ozone monitoring equipment. Recent advances in technology now allow us to measure ground-level ozone from space.

One example is a new NASA satellite mission called TEMPO, Tropospheric Emissions: Monitoring of Pollution, which will revolutionize how we measure ozone and its sources. TEMPO will monitor major air pollutants, including ozone, across North America from a geostationary vantage point. In other words, it will stay at a fixed position 22,236 miles above the Earth’s equator constantly staring down at North America. Unlike most existing pollution monitoring satellites which provide once daily measurements, TEMPO will provide hourly data at high spatial resolution. In fact, TEMPO will be able to distinguish between areas of 10 square kilometers, a region a little smaller than the Los Angeles International Airport. And TEMPO isn’t alone in revolutionizing how we monitor air quality. It will be one of three instruments making up a global air quality "virtual constellation"measuring air quality every daylight hour around the Northern Hemisphere. The other instruments include South Korea’s GEMS (Geostationary Environment Monitoring Spectrometer), and the European Space Agency’s Sentinel-4. This means that many people will know the quality of the air in their neighborhood.

Global Air Quality Contellation Advisory

Ozone sensitive plants can help detect ozone pollution in areas that are not currently being monitored by instruments. A public garden that includes these plants gives us an easy way to understand air quality through a connection with nature. In regions with poor air quality, you can see the plants transition from green and healthy to spotted and unhealthy over the course of the summer. When visiting ozone bioindicator gardens in our network, you can observe and record damage to ozone sensitive plants within the garden, helping scientists to better understand the causes and consequences of ozone pollution to plants.

Curious if ozone concentrations in your neighborhood are high? Ozone bioindicators may be more common than you think, and you can plant some in your own yard. Try planting one of the known ozone bioindicator species identified by the U.S. National Park Service. You can also check out your nearest EPA ozone monitor if you live in the U.S.

If you have a public space and would like to join our garden network, please fill out this Google form.

Ozone Garden Locations Map

Browse the various ozone garden locations by using the map below. You can click on a leaf for more information and to view the data collected at that location.


We would like to thank the Green Team from CU Boulder for their hard work and dedication in the development of this Ozone Garden website:

Kara Wallace, Michael Whitlock, Tanner Slemmer, Abbi Nicholson, Hunter Belcher, Percy Bell, Tanya Leung

2021 website updates done by the No-Zone Team from CU Boulder:

Haley Drexel, Nick Volpe, Sabrina Kavesh, Travis Cochran, Tyler Devlin, Yu Li