These pollutants – dubbed "obesogens" as a result of these findings – are so ubiquitous that almost everyone now has them in their bodies. Ninety-five per cent of Americans excrete BPA in their urine; 90 per cent of babies have been found to be exposed to phthalates in the womb; and every umbilical cord analysed in the new Spanish study was found to contain organchlorine pesticides such as HCB.
Two American studies have implicated phthalates in obesity in adult men, but the new research is much more conclusive, and is the first to show the effects of exposure in the womb, where humans are most vulnerable.
Dr Pete Myers, one of the world's leading experts on obesogens, told The Independent on Sunday last night: "This is very important. It is the first good study of the effects on the foetus. Its conclusions are not surprising, given what we know from the animal experiments, but it firmly links such chemicals to the biggest challenge facing public health today."
No one knows how HCB causes obesity. The Spanish scientists speculate that it may have made the mothers diabetic, which would increase the chances of their children becoming obese (see graphic, above).
Dr Myers, who is chief scientist at the US-based Environmental Health Sciences, which helps to increase public understanding of emerging scientific links, says this is "plausible", but adds that the animal experiments point elsewhere. These have shown that obesogens "switch genes on and off" in the womb, causing stem cells to be turned into fat cells. The children then grow up with a much greater disposition to store and accumulate fat.
Whatever the explanation, the research goes some way to undermining David Cameron's assertion in a speech this summer that obesity is purely a matter of "personal responsibility", a view echoed by his health spokesman, Andrew Lansley 10 days ago. The Tory leader said that the obese are "people who eat too much and take too little exercise".
Dr Myers calls that "wishful ideological thinking which does not accord with biological reality", adding: "We need to discover ways to reduce exposures to these chemicals so that changing diet and lifestyle has a chance to work."
Factors that may pile on the pounds
Why is the world getting so fat? Everyone agrees that people gain weight by taking in more calories in their food than they burn off through everyday activities and exercise. But many scientists are coming to believe that changes in diet and exercise do not sufficiently explain the rapid growth of the epidemic. As 'The Independent on Sunday' reported last week, there has been no reduction in physical activity in Britain since 1980, while obesity rates have quadrupled.
The genetic make-up of a population does not change rapidly enough to provide an explanation. So the hunt is on for other factors that might show why more people are gaining weight more easily.