California and Oregon are America’s hotspots for dangerous air toxins that may lead to autism, a study suggests.

Researchers at Washington University of St Louis collected 25 years’ worth of data on submicron (PM1) air pollution, particles less than one millionth of a meter wide and one-sixth the size of human blood cells. 

While scientists have long tracked the health effects of its cousin fine particulate matter (PM2.5), emitted in the air via fossil fuels, PM1 has largely flown under the scientific radar. 

Though little-known, PM1 has been shown to increase the risk of heart attacks, lung cancer and dementia.

Health agencies are reportedly conducting a series of studies to find out if environmental pollution can lead to autism, which has surged in the US over the last two decades. 

In the new study, a map reveals concentrations of PM1 were highest in the Los Angeles, Phoenix and Portland areas, along with parts of Idaho, New Mexico and Texas. 

Less densely populated states without crowded cities, such as Colorado, Montana, Utah and Wyoming had the smallest concentrations.

The researchers did not provide exact numbers, but the data suggests larger cities in the western US along the west coast and south are the most at risk of harmful pollutants like PM1. 

A new study has measured the nationwide levels of PM1, a pollutant that has been associated with a higher risk of autism (stock image) 

The above map shows levels of PM1 based on data from 1998 to 2022. Parts of California, Oregon and Arizona had the highest concentrations

They also said the new report could be one of the first to look specifically at PM1. 

Jay Turner, study co-author, said: ‘When EPA first promulgated a fine PM air quality standard in 1997, there was considerable discussion about regulating PM 1 or PM 2.5. 

‘For numerous reasons, including but not limited to the lack of health impact studies for PM1 compared to studies for PM2.5, the latter was chosen. 

‘This study provides a comprehensive, nationwide dataset to examine PM1 impacts on health.’

California, which had higher concentrations of PM1, also has one of the highest rates of autism in the country, according to the CDC’s latest data.

An April report from the agency found one in 19 children in California have been diagnosed with autism, 48 percent higher than one in 31 nationwide. 

However, the report only looked at children ages four through eight, so it’s unclear how many older children and teens were diagnosed. 

Nationwide, autism rates have surged from about one in 150 in the early 2000s. 

Health secretary Robert F Kennedy Jr announced earlier this year a series of studies that will look into ‘environmental toxins’ he believes could be causes, including pesticides and food additives, and promised ‘there will be an answer for the American people’ by September.

Particulate matter has been shown in recent studies to raise the risk of autism by triggering inflammation in nerve tissue and possibly passing from mom to placenta to fetus in utero. 

In the new study, published Monday in The Lancet Planetary Health, researchers calculated PM1 levels based on measuring seven components found in PM2.5: sulphate, ammonium, nitrate, organic matter, black carbon, dust and sea salt.

These pollutants can come from natural disasters like volcanic eruptions, wildfires and dust storms, as well as human-generated sources like construction sites, burning fossil fuels and vehicle emissions. 

Chi Li, first study author and research assistant professor, said: ‘Putting the seven species together, we can calculate the total PM1 concentration over the country.’ 

PM1 levels were calculated based on biweekly estimates from 1998 through 2022. 

In addition to areas like California and Oregon, parts of the Midwest and eastern US like Illinois, Ohio and Pennsylvania also appeared to have higher concentrations of PM1, according to the map.

The researchers said natural disasters like wildfires may have contributed to elevated levels. A 2020 study found the area burned by wildfires in California has doubled in the last 20 years compared to the two decades before that. 

More urban areas like Los Angeles and Portland also produce more PM1 due to higher traffic, construction and a greater concentration of densely packed buildings.

Randall Martin, a professor of energy environmental and chemical engineering at WashU, said: ‘These data offer new information to advance understanding of how to improve air quality and health.’