Could autism be linked to the climate? In the US, the states with the lowest rates of autism are in the southern parts of the country, while the highest rates are in the north. This paradox inspired a new study that collected rainfall data across California, Oregon and Washington.

The rates of autism were highest among children in places with the greatest rainfall. According to the research published in the Archives of Pediatrics and Adolescent Medicine, the effect of the rainfall appeared in children who had grown up in their first three years in a wet climate.

However, the link may not be directly related to rainfall. It could be, for instance, that children in wet weather are forced to spend more time indoors. This could affect their development, perhaps by exposing them to household chemicals, or from a lack of vitamin D, which is made in the body when exposed to sunshine.

“There is also the possibility that precipitation itself is more directly involved,” the researchers wrote. “For example, there may be a chemical or chemicals in the upper atmosphere that are transported to the surface by precipitation.”

Polluted air and rainfall may lead to other diseases. Scientists at Salford University found that humidity and pollution in the atmosphere together may trigger outbreaks of meningitis. The most infectious type of the disease, meningitis B, is closely linked to weather conditions and poor air quality. And the high level of humidity in the climate may also play an important part in the high incidence rates of asthma and chronic obstructive pulmonary disease in Britain.