In New Ozone Alert, a Warning of Harm to Plants and People

Scientists are still trying to unravel the damaging effects of ground-level ozone on life on earth. But as the world warms, their concerns about the impact of this highly toxic, pollution-caused gas are growing.

For the last four years Jack Fishman, a professor of meteorology at St. Louis University, has guided the planting of five gardens in the Midwest, gardens that have a distinct purpose: to show the impacts of an invisible gas that is damaging and contributing to the premature death of forests, crops, and other plants — and also humans. 

“The idea of the ozone garden is that it is a canary in the coal mine,” said Fishman, who recently planted one of the gardens at the Missouri Botanical Gardens. The snap beans, milkweed, coneflowers, and other plants that turn brown and sickly from exposure to ozone when it occurs at ground level, he said, are “natural bio-indicators showing this pollutant is harmful to anything that lives — a human, a squirrel, or plants. It’s the only way to show the real-time impacts of something that’s happening to our planet.” 

Leaves of a snap bean plant that is sensitive to ozone [top] compared with one that is ozone tolerant.

Leaves of a snap bean plant that is sensitive to ozone [top] compared with one that is ozone tolerant. Jack Fishman/SLU

The ozone crisis may fly under the radar compared to climate change, but it is entwined with that mother of all problems. And it’s nothing new for Fishman. He began studying ground-level ozone for NASA in 1977 and has been warning about its impacts since the 1980s. Ozone is highly toxic, even at very low concentrations. Over the last few decades, background levels have steadily increased, though pollution controls have brought down the highest concentrations in many places. And while what scientists know about the effects of ground-level ozone on life on earth is deadly serious, the subject is not all that well-researched — and what they don’t know is potentially disastrous. 

Ozone is both a naturally occurring and human-created gas. It is chemically similar to chlorine; at high enough concentrations, it has an odor and is a pale blue color. Copy machines give off the smell as they heat up. Because it is not as pungent as chlorine, ozone is used as a less harsh alternative to chlorine for disinfecting swimming pools. 

Ozone is created by humans, most abundantly when gasoline is not burned completely in cars, trucks, airplanes, cargo ships, and anything else with an engine that emits exhaust containing carbon monoxide, nitrogen oxides, and hydrocarbons. These fugitive precursors go through chemical reactions in the atmosphere, driven by warm temperatures and sunlight, to become ozone. Ozone is also created naturally from volatile substances emitted by lightning, soil, and vegetation. Theterpenes, which cause the smell of pine trees, for example, are hydrocarbons that become ozone through a chemical reaction. . 

A major and growing source of ozone in the United States is oil and gas fields. There are 1.5 million oil and gas wells in the U.S. and ethane, butane, methane, and propane are among the ozone precursors released from pipes, inadvertently and purposefully, by oil and gas companies. Over the last 40 years, ethane levels had dropped by 60 percent. But a study published earlier this year in the journal Nature Geoscience found that much of that drop had been reversed in the last five years by emissions from oil and gas wells. “If this rate continues, we are on track to return to the maximum ethane levels we saw in the 1970s in only about three more years,” said Detlev Helmig, an atmospheric scientist at the University of Colorado, Boulder and an author of the study. 

As a greenhouse gas, ozone contributes to global warming. Yet unlike C02 or methane, it is toxic. And unlike those greenhouse gases, fortunately it’s short lived, lasting a few months in the atmosphere as opposed to centuries for CO2. That’s long enough, however, to do a lot of damage. 

Ozone has gotten a lot less attention than it deserves, experts say. That’s largely because there’s a perception problem. Most people think of it solely as a good gas, the essential ingredient in the ozone layer where, high above the planet in the stratosphere, it shields us from harmful ultraviolet light. 

An MIT study estimated that ozone causes between 4,700 and 19,000 deaths annually in the U.S.

But the saying among researchers is, “Good up high, bad nearby.” When ozone occurs at ground level, it’s a serious problem — a poison that people, plants, and animals take in, and when the levels are high enough, it weakens and even kills. A 2013 study from the Massachusetts Institute of Technology estimated that ozone, which exacerbates heart and lung problems, causes between 4,700 and 19,000 deaths annually in the U.S. 

The most egregious levels of ozone in the U.S. are in and near urban areas. The Environmental Protection Agency has focused its efforts there and recently proposed to lower the limit from 75 to 70 parts per billion (ppb), which industry has fought. There has been good progress made since the 1960s and 70s when ozone reached levels of 200 ppb or more, especially in Sun Belt cities such as Los Angeles and Houston. 

But while urban ozone is a fairly well known problem, far less well known is that global background levels of ozone are increasing. Before the industrial era, naturally occurring ozone levels were about 10 ppb. In many places outside of urban areas in the U.S. these days, the ambient levels of ozone have gone from 40 ppb in the 1980s, where they start having visible impacts on vegetation, up to 50 ppb and higher. Part of the reason is that ozone travels long distances and elevates levels far from its source. People are also spreading out further and further and moving into what were once rural areas. 

These background ozone levels were seen as a serious problem as far back as the 1980s. Fishman, who researched the issue for NASA at the time, even published a 1990 book called Global Alert: The Ozone Pollution Crisis, which described what was happening to forests, crops, and human health because of background levels of the gas. Another major warning came in 2008 when a team of experts published a report for the Royal Society titled Ground Level Ozone in the 21st Century: Future Trends, Impacts, and Policy

Maximum daily averages of background ozone in parts per billion in California and Nevada.

Maximum daily averages of background ozone in parts per billion in California and Nevada. NASA/JPL

Yet nothing has happened to significantly change the trajectory. Ozone is rising around the world, and more damage is being done. And the Intergovernmental Panel on Climate Change estimates that global ambient levels will go up substantially over the next couple of decades. 

One thing that causes alarm among researchers is how little is known. Human health effects are well studied, as are the impacts on agriculture. Potato farmers in coastal U.S. Mid-Atlantic states had to stop growing Le Chipper potatoes, which make especially good potato chips, because ozone was killing the plants. Legumes, such as soybeans and peanuts, are especially sensitive to ozone; studies show that the yield of soybeans is reduced by 10 percent when exposed to high levels of ozone. And a strain of tobacco for cigar wrappers can no longer be grown because ozone kills it. Globally, it is estimated that ozone takes a $20 to $70 billion toll on crops annually. 

Far less is known about wild plants. There is an ozone garden in Great Smoky Mountains National Park, and visible damage there has been documented on coneflowers, milkweed, and other wild plants. But Art Chappelka, a professor of plant pathology at Auburn University who has long studied the impacts of ozone on forests and other aspects of ecosystems, says precious little research has been done on the impacts of ozone on forests, other wild plants, and wildlife. Experts are still largely in the dark on what is going on in the woods, although they know that the impacts are serious. 

“Visible injury [to plants] in the Smokies have gone down, ozone levels have gone down” largely because of reductions in emissions from nearby power plants, Chappelka said. “But what does that mean? Are there still effects? We just don’t know. The forests could be recovering, or they could be in long-term decline. We just don’t know.” 

Then there is the combination of problems that can gang up on an ozone-weakened forest. “We have the interaction with climate,” said Chappelka. “Drought is a big problem. What if insects are more prevalent because of the climate? We do know that ozone is another stressor on plants and can predispose them to insects and certain diseases.” That may be one of the causes of recent outbreaks of disease and pests in forests around the world.

Forests are difficult to study because there are few controls — i.e., populations that aren’t impacted. But studies conducted in enclosed chambers show several effects. One of the biggest impacts is that trees release more water than they should. “Ozone seems to paralyze the stomata,” which are pores in tree leaves that regulate the release of water vapor, said Howard Neufeld, a plant ecophysiologist at Appalachian State University in North Carolina. That means trees transpire more water into the atmosphere than they would otherwise. One study in Tennessee showed that so much additional water was leaving ozone-impacted trees that they took up more water from the ground to compensate. That deprived an entire watershed of its usual supply and reduced stream flow. 

Ozone also shortens the time that leaves stay on trees, which means less food for the trees. That reduces stem and root growth, which makes them more susceptible to drought and insects. In one study, Neufeld found that milkweed exposed to high levels of ozone was losing its leaves a month earlier than other plants. 

Ozone also causes global warming indirectly, by reducing the amount of CO2 that trees take up.

Trees also reduce their photosynthesis due to ozone. That makes it a double whammy for climate change — as a greenhouse gas, ozone directly causes global warming; but it also does so indirectly, by reducing the amount of CO2 that trees take up. 

The overall impact of ground-level ozone on biodiversity is a big unknown, especially when combined with climate change, C02, and anthropogenic nitrogen. Because it limits plant species that are sensitive to it, the Royal Society report found, ozone may have “long-term effects on ecosystem structure and function.” Chappelka has done some work feeding rabbits vegetation laced with high ozone levels, but does not yet have any results on how it might be affecting them. 

Chappelka said one of the great unknowns about the impact of ozone is what it means to the gene pool of a particular species. Some individuals or communities of a species might be susceptible to ozone, he said. “Are we eliminating those? Are we changing the genetic structure, and instead of a very diverse population, the population is becoming less diverse?” 

Climate change comes into play another way. In longer growing seasons, when ozone is at its worst because of high temperatures, plants take up more. And hotter weather or longer periods than average with high temperatures increase the levels of ozone in the atmosphere from both natural and human-caused sources. 

A major area of study these days is the international transport of ozone. While Los Angeles and other U.S. cities have come a long way in cleaning up their air in recent decades, Beijing, New Delhi, and other cities in Asia have gotten far worse because of rapid growth, a lack of pollution controls, and the burning of biomass — such as forests and crop stubble — which is another major contributor to ozone. Some of that ozone is fouling the air on the U.S. West Coast. Studies show that Asian ozone adds as much as 15 ppb to high elevation cities in the West, a substantial part of their federally mandated limit of 75 ppb. 

As Asian ozone makes it way around the world, it is being blamed for damage to forests from South and North Korea, to Japan, to the U.S. In southern Europe, meanwhile, there is concern about the impact of ozone on the forests of Portugal, Spain, Italy, and France because of high temperatures there. 

Solving planetary ozone levels is a thorny problem. Levels in the eastern U.S. have come down substantially over the last 20 years, thanks largely to pollution controls on power plants and automobiles. 

Unfortunately, in a warmer world, climate change mutes the effect of those reductions by creating more ozone from natural and human-made sources as temperatures get hotter. “If the climate continues to warm,” said Neufeld, “we’re going to have to clean up the environment even more, because otherwise we’ll make more ozone faster.” 

For Fishman, who started his research 40 years ago, it has been frustrating. “There should still be a global alert about ozone,” he says, “in fact, more than ever.”