Babies whose first few months of life coincide with high pollen and mould seasons may be at increased risk of developing early symptoms of asthma, research suggests.
Children born in autumn and winter were three times more likely to start wheezing by the age of two than those born at other times of the year.
In the region of California where the study took place, air levels of fungal and mould spores rose in November and December, while pollen peaked in the early spring months of March and April.
High concentrations of two groups of fungal spores produced by mushrooms, moulds and plant rusts, had a significant association with wheezing at age two, said the researchers.
Basidiospores and ascospores are common outdoor allergens that are more prevalent during periods of rain or high humidity.
Exposure to pollen during the first three months of life was also linked to a greater risk of wheezing. Pollen from alder, pine and cypress trees had more of an effect than that produced by oak, mulberry or elm trees.
The findings were reported online today in the journal Thorax.
Study leader Dr Kim Harley, from the University of California at Berkeley, said: "Until our paper, there were very little data about exposure to allergens in the air, which we know can trigger symptoms for those who already have asthma. This is the first study to look at the potential role of early life exposure to multiple outdoor fungal and pollen groups in the development of asthma."
Asthma is not normally definitively diagnosed until children are around school age, said the researchers.
As many as 40% of children who wheeze early in life may go on to develop childhood asthma, especially if they have other allergic symptoms.
The scientists are continuing to monitor the progress of the children taking part in the study.
Co-author Dr Ira Tager, also from the Berkeley campus, said: "We are not in a position to say conclusively why some children develop asthma, or to even suggest precautionary measures to help babies born in the fall and winter. We already know that family history is a major risk factor for developing asthma, but the role environmental factors play is still being fleshed out. What this study does is provide valuable clues about airborne allergens that are worth exploring further."