A new study has found more evidence that kids are less likely to develop asthma if they grow up in a home with a dog or a farm animal.

Researchers have long noted that kids who aren’t exposed to many microbes in their early years are more likely to develop asthma and allergies, a theory known as the “hygiene hypothesis.”

Previous research has linked pet ownership to a reduced risk of asthma in kids, but this new study is considered the largest of its kind.

It found that kids who grew up with dogs have an approximately 15 per cent lower risk of asthma than kids without dogs, the yreport in the journal JAMA Pediatrics.

Tove Fall of Uppsala University in Sweden led the research team. She says they looked at more than 1 million children born in Sweden from 2001 through 2010, as well as their prescription medicine history to see which ones sought medication for asthma.

They then cross-referenced that information with Sweden’s dog-licence and farm animal registries, so they could determine which families had animals in the first year of a child’s life.

They found that school-aged kids exposed to a dog during their first year of life had a 13 per cent lower risk of asthma. Farm animal exposure was associated with a 52 per cent reduced risk of asthma in school-aged children, as well as 31 per cent reduced risk in preschool-aged children.

Fall said, because her team had access to so many details about such a large group of kids, they could account for other factors that might have interfered with the results, such as asthma in parents, area of residence, or socioeconomic status.

The researchers noted their study could only find a link between animal ownership and asthma, not provide answers on how animals might protect kids from asthma.

Co-author Catarina Almqvist Malmros, a pediatrician and professor in clinical epidemiology at Karolinska Institutet in Stockholm, says the study shows that kids have a reduced risk of asthma when they’re exposed to dogs or farm animals.

“We know that children with established allergy to cats or dogs should avoid them, but our results also indicate that children who grow up with dogs have reduced risks of asthma later in life,” she said in a statement.