Compared to the United States, where reports of autism have jumped 78 percent since 2007, autism rates in developing countries are growing at a crawl. But in light of new research linking autism with air pollution, experts say children in middle- and low-income countries may have a higher risk of developing the disorder than those in the United States.

In a study of 325 US women, Andrea Roberts, an associate researcher at Harvard School of Public Health, found that pregnant women exposed to high levels of air pollution are up to two times more likely to have a child with autism. Twenty to 60 percent of those evaluated lived in areas with hazardous levels of air pollutants like diesel, lead, mercury and manganese.

“It’s not 1 or 2 percent of women who are at risk,” Roberts said. “It’s a lot of women.”

Previous studies have linked autism risk with living close to a highway. Robert’s study adds weight to the evidence that air may be bad for children’s brain development. When it comes to autism, though, there is no one explanation, Roberts said. “Almost always, genetics work together with environmental factors,” she said.

Roberts said children in low- and middle-income countries may be in greater danger of exposure to toxic air pollutants than children in the US. China, India and Peru, for example, are home to some of the world’s most polluted cities, according to Blacksmith Institute, an international nonprofit dedicated to solving pollution problems.

“In many of these countries, they haven’t tackled any of these pollution types,” said Richard Fuller, president of Blacksmith Institute. “They have no Clean Air Acts. If they do, there’s very little enforcement, no monitoring.”

Blacksmith Institute has done extensive research on the health effects of chemicals like lead, mercury, arsenic and chromium in air, soil and water. In a recent study, Fuller found 189,725 children in seven lower income Asian countries were at risk for “diminished intelligence” because of exposure to lead.

Another report released this month by the Global Alliance on Health and Pollution estimated as many as 200 million people living near 3,000 highly polluted sites in several countries, including India and the Philippines, are at risk of disability. The report, funded by the World Bank, the European Commission and the Asian Development Bank, concluded that the burden of disease associated with pollution is comparable to that of malaria. 

“We know a lot about lead poising, which is linked to intellectual impairment,” Fuller said. “But this new analysis adds a whole other level of public health alarm to these toxins.”

Health experts have put forth a number of theories to explain the global differences in autism prevalence rates. Roberts and Fuller both cited the most common.

Because of health systems — particularly mental health systems — are less developed, it’s “very likely,” Roberts said, that autism is undercounted in lower income countries.

“Autism is not the easiest thing to diagnose,” Roberts said. “You don’t just look at a child and know he has autism. You observe them. You ask them a bunch of questions.”

The United States has more psychiatrists than the world’s two most populous countries and the continent of Africa combined, according to a 2009 report from the London School of Hygiene and Tropical Medicine. Ninety percent of people struggling with mental and neurological disorders living in low- and middle-income countries go untreated.

“The efforts of the health sector go toward fighting infectious disease,” Fuller said. “There’s not much conversation on these environmental threats.”  

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