A new Irish study has shown a major link between air pollution and the development of childhood asthma.

Researchers at University Hospital Galway examined the relationship of air pollution and climatic variables to asthma admission rates of children in Galway city over a 21 year period - 1985-2005.

Over the past 20 years in Galway, asthma admission rates increased steadily to a peak in 1995 and then dropped dramatically. The study indicates that the drop in asthma cases is linked to improved air quality in recent years.

Data collected for the study was obtained by recording child asthma admissions from 1985-2005 and admission rates per thousand calculated for pre-school, school-aged and all children (1-14 years) on a monthly and annual basis. Children under one year of age were excluded from the study

The data was compared to average monthly and annual weather conditions (rainfall, humidity, sunshine, wind speed and temperature) and black smoke levels for the city.

The study found evidence of a positive association between asthma admissions and black smoke levels for the one to four-year-olds and a positive trend for 5-14 year-olds separately.

There appeared to be a stronger association between black smoke and admissions rates in ages 1-4.

There was evidence of a positive association between asthma admissions and smoke concentrations for all subjects (1-14 years).

The authors concluded there was a strong association between black smoke levels and admission to hospital with childhood asthma.

The researchers say that the reason for the rise and fall in black smoke from the mid-nineties is probably a combination of changes in domestic and vehicular fuel use.  Sale of bituminous coal was banned in 2000 and this was associated with a further small reduction in smoke levels.

Therefore some of the fall in smoke levels from 1995 is due to 'cleaner' coal, the authors say.

The rise in smoke levels up to 1995 coincided with a growth in registered vehicle numbers in the county (from 44,000 to 108,000) and the proportion of diesel powered vehicles (from 18% to 35%), indicating that vehicular traffic was a factor in elevating smoke levels, the authors say.

In conclusion, the authors say there is an an association between childhood asthma levels and black smoke in Galway city.

They say regulatory changes in the nature of coal and diesel, and more stringent vehicular emission rules have improved air quality and contributed to the reduced incidence of hospital admissions.

The research is published in the Irish Medical Journal.