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."