California’s air pollution levels are among the worst in the country, and climate change is making the situation worse, according to a new report from the American Lung Association.

Despite the state’s efforts to reign in air pollution, 90 percent of California residents are exposed to unhealthy air at some point during the year, according to the State of the Air 2018 report. The study tracked smog and air particle pollution levels across the nation between 2014 and 2016.

Los Angeles topped the list of smoggiest places in the country, as it has for most of the past 19 years. Bakersfield suffered the highest levels of short-term particle pollution in the nation. Overall, the San Joaquin Valley dominated air pollution rankings, with Visalia, Bakersfield and Fresno all landing among the top five regions with most short-term and year-round particle pollution.

While air pollution is bad for everyone’s health, low-income communities, children, seniors, and people with underlying health problems tend to be most impacted when ozone and particle pollution levels soar, experts said.

Afif El-Hasan, a pediatrician in Orange County, said the poor are more likely than wealthier people to live close to freeways, railways, factories, transportation hubs and other air pollution sources because rents and home prices in those areas are cheaper.

Low-income people face additional hurdles when it comes to protecting themselves from air pollution, he explained. They may have to keep their windows open on smoggy days because they can’t afford air conditioning, and they may have to ride bikes or stand at bus stops on smog-choked streets because they don’t have a car. Often, poor people also have a harder time accessing preventative care, he said.

“You end up with this population having disproportionately more illness, more lung disease, more heart disease, more ER visits, more time off of work, more school days lost for kids,” he said. “There are a lot of issues that go along with it, and it kind of snowballs, unfortunately.”

The health impacts of air pollution are particularly grave for children, El-Hasan added. Children who grow up in polluted areas have been shown to develop less lung capacity than children who live in healthy air, he said. That can leave them more vulnerable to lung problems as they age, he said.

“That is a horrible thing to do to children,” he said. “It’s not fair.”

California has unique air pollution challenges because of its large population, temperate climate and geography that can cause bad air to accumulate in certain areas, said Will Barrett, a senior author on the report. However, climate change is exacerbating the state’s air pollution woes by driving up temperatures—leading to more summertime smog—and provoking more wildfires and drought.

“The climate change conditions that we’re seeing are creating these extreme weather events that are ultimately, unfortunately, becoming the new normal in California,” he said. “We have to redouble our efforts to cut down on all harmful pollutions: ozone particle pollution, carbon pollution and local air toxins that threaten health.”

Barrett said clean air policies in California are making a difference. But he cautioned that federal efforts to curb environmental regulations, and budget cuts at the U.S. Environmental Protection Agency, could threaten progress that’s been made toward reducing air pollution.

“We have to be on alert,” he said. “We have to continue to support our California programs and make sure that our federal policies are kept in place, and that our agencies that are supposed to protect public health are doing their work.”