Thumb-sucking, nail-biting can actually keep allergies at bay

The findings showed that children with both thumb-sucking or nail-biting habits were less likely to be allergic to things such as house dust mites, grass, cats, dogs, horses or airborne fungi.

"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," said Malcolm Sears, professor at McMaster University in Ontario, Canada.

"While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits," Sears added in the work published in the journal Pediatrics.

In the study, the researchers tested the idea that these common childhood habits would increase microbial exposures, affecting the immune system and reducing the development of allergic reactions also known as atopic sensitisation -- the tendency to be "hypoallergenic”.

The habits were measured in a longitudinal birth cohort of more than 1,000 New Zealand children at ages 5, 7, 9 and 11. Atopic sensitisation was measured by skin-prick testing at 13 and 32 years old.

The researchers found 31 per cent of children were frequent thumb suckers or nail biters.

Among all children at 13 years old, 45 per cent showed atopic sensitisation but among those with one oral habit, only 40 per cent had allergies. 

Among those with both habits, only 31 per cent had allergies. 

This trend was sustained into adulthood and showed no difference depending on smoking in the household, ownership of cats or dogs or exposure to house dust mites.

However, the study did not find associations between the oral habits and development of asthma or hay fever, the researchers noted.