London - Wearing outdoor shoes in the house could reduce the chances of your children getting asthma.
Scientists who tracked more than 1 400 youngsters found the risk was greatly reduced in city homes that had similar levels of dirt to farmhouses.
The results suggest that scrupulously clean urban homes could benefit from the soil bacteria that train immune systems to resist asthma.
Pirkka Kirjavainen, who led the study at the National Institute of Health and Welfare in Finland, said: "It appears that it is important for children to be exposed to less human bacteria and more soil bacteria.
"The next step is to put bacteria from farms on to the floor in urban homes and see if children who crawl along this floor are less likely to get asthma. We might be able to develop a nasal spray of helpful bacteria for children, but we will need to establish the best mix of bacteria and safety first."
The results, which were reported in the journal Nature Medicine, suggest that farm-like bacteria can reduce a child’s risk of getting asthma by up to seven times.
Children who did not live on farms were less likely to get asthma if they grew up in homes with similar bacteria to farms – which was more likely if the occupants wore outdoor shoes inside, the house was older and there were more children in the family.
Dr Kirjavainen added: "We have known for more than 20 years that children who live on farms are less likely to get asthma and allergies. But we cannot all live on farms, so it is good news to have found this 'farmyard effect' can be taken outside the farm and bacteria can provide the same protection."