As Houston reaches its fifth week of searing temperatures, residents may have more than just the heat to worry about. 

Ozone pollution, a smog-like substance produced by the mixture of nitrogen oxides and volatile organic compounds, is causing unhealthy air quality. These pollutants, which come from cars, power plants, refineries and chemical plants, combine with high heat and sunlight to create ground-level ozone. 

So far this year, the Texas Commission on Environmental Quality has declared 18 high ozone days in the Houston metropolitan area. That means the level of ozone pollution is unhealthy, very unhealthy or hazardous. It has declared 204 high ozone days across the Houston-Galveston-Brazoria region this year.

Houston had 30 high ozone days in all of 2022, with only seven by the end of June. The region totaled 101 last year.

The TCEQ will host a public hearing Tuesday night to discuss the state’s plan to lower ozone-causing emissions.

Ozone can cause respiratory problems, such as difficulty breathing, lung infections and inflamed chests, said Inyang Uwak, the research and policy director at the nonprofit Air Alliance Houston. While healthy people can react to ozone, those most at risk are children, elderly adults, people with respiratory issues, including asthma, and those who work outdoors. 

“As you can imagine, in Houston we have a lot of heat and a lot of sunlight, which combines with all our industry and cars,” Uwak said. “It all makes Houston a unique spot for high ozone levels.” 

Houston has the ninth highest level of ozone pollution in the United States.

The Houston-Galveston-Brazoria area is listed as being in moderate violation of the EPA’s ozone pollution regulations. That is the EPA’s second-lowest classification, but up from the region’s previous listing of marginal nonattainment. 

Loren Hopkins, chief environmental science officer for the Houston Health Department, blamed weather patterns for the increase in high ozone days.

“It’s not just that it’s hot. It has to do with cloud cover and the (ultraviolet) rays from the sun and if it’s going to rain or not,” she said. “You could look back at some previous July and see that it was really hot, but maybe it didn’t have that many ozone action days because of cloud cover or rain.” 

Hopkins said ozone action days used to be easier to predict, but climate change-caused weather fluctuations have made it harder to determine what months or times during the year will have high levels. 

The chemical precursors to ozone pollution, such as volatile organic compounds and nitrogen oxides, also are triggering for people with respiratory issues, even without ozone fully forming. 

“When the high ozone alert is going off, that means that there has been several days beforehand where VOCs and NOx have been building up,” Hopkins said. “Formaldehyde is a precursor to ozone, and we know that parts of the city have elevated levels of formaldehyde.”

In a 2021 report, the Houston Health Department published a report that found elevated levels of formaldehyde – a known carcinogen and air pollutant – in communities along the Houston Ship Channel. 

“We’re really going to have to double-down on our reduction of emissions so we can reduce our ozone pollution further,” Hopkins said. 

The Houston area for years has fallen short of meeting ozone pollution regulations set by the Environmental Protection Agency. In 2022, the area was reclassified as being in moderate violation of the EPA standard set in 2015. The EPA initially set those standards to protect public health from the adverse effects of ozone pollution. 

To meet EPA requirements, state regulators must create a new state implementation plan, or SIP, to lower the Houston-area’s emissions. In May 2023, the TCEQ commission approved a proposal for the Houston area’s revised SIP since moving into moderate violation. That will include a budget for how much cars, trucks and buses can emit in Houston. 

The public comment period on the SIP lasts through July 17. Tuesday’s public hearing is at 7 p.m. at the Houston-Galveston-Brazoria Area Council, 3555 Timmons Lane #100. Residents can leave oral or written comments. 

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Elena Bruess covers the environment for the Houston Landing. She comes to Houston after two years at the San Antonio Express-News, where she covered the environment, climate and water. Elena previously...