Oil refineries and chemical plants are 24-hour operations. They’re not designed to be turned off. But when a hurricane barreled through the refinery and chemical plant capital of the United States, many didn’t have a choice.

When refineries are forced to shut down—as were at least 11 along the Texas coast and the greater Houston area, due to Hurricane Harvey—they often release far greater volumes of toxic air pollution than the normal legal limits would allow. In industry parlance, these pollution spikes are called “exceptional events.” The excess pollution is considered an emergency necessity to prevent worse outcomes, like an explosion, so plants are exempt from fines they would ordinarily pay for exceeding their legal pollution limits.

In Texas, residents who live near plants closing down due to Harvey watched as they began flaring off their excess chemicals. Almost half of the country’s petroleum and natural gas refining capacity sits along the Gulf Coast, and Houston, 30 miles inland, is home to the largest refining petrochemical production complex in the country. Around 16% of US refining capacity has been interrupted by the storm.

Air quality monitors in Houston, where much of the excess pollution is taking place, have been shut off during Harvey, as The New Republic’s Emily Atkin reported:

Thankfully, according to Daniel Cohan, an associate professor of environmental engineering at Rice University who specializes in air pollution, the storm will likely prevent common forms of excess pollution, like particulate matter, sulfur dioxide, and nitrogen oxides from sticking around long; For example, any excess nitrogen oxides probably won’t have a chance to form into harmful smog, because “you need heat to cook that up,” says Cohan, and the weather is still stormy.

Other pollutants, like particulate matter, will likely be disbursed quickly: “Fortunately it is happening at a time when it’s been raining nonstop. With that rain, you’ve got turbulence—it won’t be sitting stagnant in one place. And you also have a lot of people who can’t go outdoors. That would lessen the exposures,” Cohan says. “If there was ever a time to have these releases, this is [it].”

But he is concerned about other types of air toxics reaching the neighborhoods closest to the plants, and which have been exposed to high levels of pollution for years. Residents in the largely Latino communities that live in Houston’s East End, closest to the industrial complexes where a large portion of the nation’s refinery activity takes place, reported “unbearable” petrochemical smells drifting into their neighborhoods during Harvey. These neighborhoods, sometimes within a mile of refineries’ perimeter fences, have long faced “more exposure to air toxics than almost anywhere in the country,” Cohan says. Household income in the area is well below the city average.

“We know that we have elevated levels of cancers all along these areas. There have been many reports to show increased rates of childhood leukemia if you live within two miles of the Houston ship channel, for example,” Juan Parras, the director of the grassroots environmental justice group TEJAS, said in an interview with Democracy Now.

Many of these neighborhoods are under several feet of water right now. “Fenceline communities can’t leave or evacuate so they are literally getting gassed by these chemicals,” Bryan Parras, also of TEJAS, told The New Republic.

Records filed with the Texas Commission on Environmental Quality show ExxonMobil shut down two of its refineries due to heavy rains; it made the decision to close its Baytown refinery, the second largest in the country, after a floating roof covering an oil tank sank in the floods. When Baytown was shut down, it released about double the amount of volatile organic compounds—a broad category of air toxics—than its permit normally allows. Shell shut down its Deer Park refinery, releasing excess benzene, a cancer-causing toxin, as well as toluene and xylene, both neurotoxins, in the process.

Many chemical plants in the area faced a similar situation; Dow Chemical’s major operation in Freeport, Texas, on the Gulf Coast about 70 miles south of Houston, shut down after being struck by lightning during Harvey, according to its regulatory filing. It released toxins including benzene, hexane, and toluene far in excess of its permits. About 20 miles south of Houston, Equistar Chemicals’ sprawling Channelview complex lost power during Harvey, forcing it to flare off excess gasses—releasing carbon monoxide, ethylene gas, and a host of other air toxics not typically covered by its permit in the process.

Water pollution from some facilities is also spiking: BASF’s Beaumont Agro plant, which produces agricultural chemicals, notes in an “exceptional event” filing that its toxin-laden waste water “will continue to overflow to the ground until the rain stops.”

Meanwhile, several Superfund sites—areas designated the most toxic in the country—threaten to contaminate the floodwaters too. Harris County, the seat of Houston, has more Superfund sites of any county in Texas. As people wade into flood waters, experts are concerned about exposure.

Up the road from the Brio Refinery Superfund site in Harris County, where ethylbenzene, chlorinated hydrocarbons, and other chemical compounds were once pooled in pits, nearby retention ponds turned into swimming holes. “Yesterday as these large retention ponds filled up, eight feet deep in places, kids were swimming in them, and that’s not good,” Wes Highfield, a scientist at Texas A&M University-Galveston, told the Washington Post.

The San Jacinto River Waste Pits Superfund site, also in Harris County, is known to the Army Corps of Engineers to be likely to flood during storm surges. According to Juan Parras of TEJAS, the pits, contaminated with the toxic chemical dioxin left behind from an old paper mill, are completely underwater right now.

“Each time we have a rain event, this contamination is being spread into more communities, homes, neighborhoods, and further exposing more and more people,” Parras told Democracy Now.