Asthma is on the decline: - But why?

The good news is that asthma is on the decrease after rising for 40 years. The bad news is that nobody has a clue about the reason for the sudden decline.
In the UK alone, self-reporting of asthma among children aged 13 and 14 has fallen by around 20 per cent between 1995 and 2002, a trend that was mirrored by other health surveys. This phenomenon was also reported in Melbourne, Australia where incidence of asthma among the 6- to 7-year-olds has also fallen.
More studies, and larger ones too, need to be carried out before we can say with any certainty that a trend is developing worldwide, but the early signs indicate that it's beginning to look that way.
Medicine would of course like to take the credit. After all, by the end of the 1990s two-thirds of all asthma patients in the UK were taking inhaled corticosteroids. At best this could be an explanation for a drop in the number and severity of attacks, but it does not account for the falling numbers being diagnosed at the outset.
The fall also puts a question mark over other theories, such as environmental factors including air pollution, tobacco smoke, allergens, diet, and infections in early life. None can explain both the rise and fall in the disease.
So, again, when we try to see the face of illness we instead are confronted by an enigma.