Add city living to the growing list of things that might cause allergies.
Kids who grew up on a farm are much less likely to develop asthma or hayfever than their urban peers, new research has found.
Scientists looked at data collected in a respiratory health survey carried out in Europe, Australia and Scandinavia between 1998 and 2002, covering more than 10,000 people.
They were asked where they lived, the number of siblings and pets they grew up with, and other questions about their upbringing.
Farm kids were more likely to have pets and older siblings, and also more likely to share a room growing up.
Now adults, those who grew up on farms were 54 percent less likely to have hayfever or asthma than kids from the city and 57 percent less likely to have other nasal allergies.
In the study's findings, the researchers say the effects were consistent across several countries, so there must be something about the biology of farm life that helps kids grow up less susceptible to allergies, rather than cultural differences.
It adds weight to a growing consensus that exposure to microbes early in life is essential for the body's immune system to develop properly, called the 'hygiene hypothesis'.
Researchers in Finland earlier this year found wealthier, more sanitised societies suffer higher allergy rates than their poorer neighbours, where children are exposed to a wider range of microbes.
And in July, a study found Kiwi kids who such their thumbs and bite their nails grow up less likely to develop allergies.
Finally, earlier this month scientists in the Netherlands found a link between allergies and the over-prescription of antibiotics.
This latest study out of Australia is believed to be the first to look at the effects of farm life as a pre-schooler on their susceptibility to allergens as an adult.
"The 'farm effect' has been observed by a number of studies, the majority of which have focused on childhood farm exposure and disease onset in childhood, while fewer studies have investigated the impact of early-life exposures on adult disease phenotypes," the study notes in its introduction.
"Exposure to increased loads of microbes such as viral, bacterial and parasitic agents associated with farming environments has been proposed as contributing factors in this link."
The study was published today in online journal Thorax.