Prince Charles has been campaigning on saving the planet in response to becoming a grandfather - ironically his daughter-in-law could be vulnerable right now to a common environmental hazard which has just been linked with harm to the brain of her unborn child.

A team of researchers based at the Children's Hospital of Los Angeles and the University of Southern California, report that mothers who live near higher road traffic pollution during their pregnancy, and for the first year of their baby's life, suffer elevated risk of later autism in those children.

Children with autism are more likely to live at residences that have the highest exposure to traffic-related air pollution, during gestation and the first year of life, compared with children whose development is 'normal'.

The study compared 279 children with autism and 245 children with typical or 'normal' psychological growth. The mother's address was used to estimate exposure to traffic-related pollution for each phase of pregnancy and first year of life. Regional air pollutant measures were based on the Environmental Protection Agency's Air Quality System data. Pollutant levels for children with autism and for those with more typical development were compared.

Autism spectrum disorders are characterized by difficulties in communication, social interaction combined with repetitive behaviours, or restricted interests. The incidence rate of all autism spectrum disorders has been climbing dramatically in recent years, yet no one knows why. Some blame over-diagnosis, others believe this is a genuine and alarming rapid increase. The rate in the USA has now reached as high as one in 110 children, or almost 1% of all children born today.

Air pollution has begun to emerge as a potential risk factor for autism. For example, a 2006 study identified an increased risk of autism based on exposure to diesel exhaust particles, metals (mercury, cadmium, and nickel), and chlorinated solvents in Northern California. Another investigation in 2010 also reported associations between autism and air toxicology at the birth residences of children from North Carolina and West Virginia. High levels of air pollutants have been associated with poor birth outcomes, immunology changes, and decreased cognitive abilities.

Heather Volk, the lead author of the latest study, led a team which had also found a similar association between the risk of autism and an early life residence around 300 metres from a freeway, in a study published last year. Traffic-related air pollutant mixture varies depending on distance from the source, returning to near-background daytime levels beyond a distance of around 300 metres (about 1000 feet). Residence within around 300 meters of a freeway at time of delivery of a baby, compared to greater than 1,400 meters, was associated with nearly a doubling in odds of having a child who later developed autism. Proximity to smaller roadways didn't show associations with autism, which is consistent with higher concentration of pollutants near major freeways, and declines in particulate matter to background levels, beyond 300 meters from a major road.

Heather Volk, Fred Lurmann, Bryan Penfold, Irva Hertz-Picciotto and Rob McConnell, the authors of this latest research, entitled 'Traffic-Related Air Pollution, Particulate Matter, and Autism' have now found children with autism were three times as likely to have been exposed during the first year of life to higher traffic-related air pollution, compared with children with more typical or normal development. Similarly, exposure to traffic-related air pollution during pregnancy was also associated with autism.

The authors of the study point out that concentrations of many air pollutants, including diesel exhaust particles are elevated near major roads, and such diesel exhaust particles plus polycyclic aromatic hydrocarbons (commonly present in diesel exhaust) have been shown to negatively impact brain function.

Dr Geraldine Dawson, chief science officer at 'Autism Speaks', in the same issue of the academic journal where the traffic-autism link finding has just been published (Journal of the American Medical Association Psychiatry), points out that in the past six years prevalence of Autistic Spectrum Disorders has increased 78% in the USA.

Dr Luke Knibbs and Professor Lidia Morawska from the International Laboratory for Air Quality and Health, Queensland University of Technology, report in a recent review paper, that fine particles found in traffic pollution are small enough to reach the brain directly. No one knows yet how autism is caused, but clues could come from what is already known of the negative impact of traffic pollution on adult brains.

Knibbs and Morawska point out just one hour of exposure to diesel exhaust produces functional brain changes in adults, while another recent study they cite, reported that levels of serum brain-derived neurotrophic factor (BDNF), an important chemical linked to normal brain development, learning, memory and possibly crucially protective from depression and dementia, did not increase following 20 min of cycling near a major road, as it normally does following exercise.

In the study on cycling near a major road, the same exercise performed in a particle-free filtered environment, led to a 14% increase in BDNF. Exposure to Traffic Related Air Pollution (TRAP) in transport environments seems to reduce exercise benefits on BDNF levels.

Knibbs and Morawska, in their review from the academic journal 'Environment International', highlight that past research indicates wearing an efficient respirator, or face mask, greatly reduces exposure to Traffic Related Air Pollution, and has been shown to have health benefits.

Knibbs and Morawska emphasise that makeshift handkerchief masks appear to offer little health protection, compared to purpose-designed respirators or face masks.

The study investigating traffic pollution and autism made allowances for social class and economic circumstances - traffic pollution was linked to autism no matter how wealthy or poor families were. In other words living in Buckingham Palace, but near major busy roads is as dangerous, according to this new research, to the brain of a foetus or newborn, as being brought up in tougher neighbourhoods.

Sceptics of these findings argue air pollution has been generally declining in the US while autism rates have been supposedly rising, plus they contend the most traffic polluted cities in the world don't necessarily seem to suffer the highest autism levels.

But, if the effect of traffic pollution on our brains turns out to be truly harmful, given it's estimated 11% of the U.S. population live within 100 m of a four-lane highway, the public health implications are profound.

Prince Charles is exhorting grandparents to become environmental campaigners; if you are swayed by this study, the next generation needs protecting critically right now.