It began back in 1989, when a doctor named David Strachan observed younger siblings were less susceptible to eczema and asthma.
He proposed it was because they were exposed to more unhygienic practices and infections, courtesy of their older brothers and sisters. His theory became known as the hygiene hypothesis (now known by several other names, including the ‘old friends’ theory).
It’s valid to wonder about this link given New Zealand’s high rates of asthma compared to overseas.
One in seven children aged 2 to 14 years and one in eight adults currently take asthma medication, Health Quality and Safety Commission data shows.
The debate has yet to be settled, according to Otago University respiratory specialist Bob Hancox, but there are strong indicators that some aspects of the hypothesis may have an element of truth. Hancox says the theory is that childrens’ immune systems get “trained” by exposure to more viruses and microbes, both in what their bodies should react to, and what they should ignore.
As well as comparisons between older and younger siblings, there are also studies (including one co-authored by Hancox) that suggest children who put their hands in their mouths when sucking their thumbs, or biting their nails, are less likely to develop allergies.
However, when it comes to asthma, no strong definite link has been made. Some studies suggest children growing up on farms, or in homes with cats and dogs, had a reduced risk of developing asthma.
Hancox says this points to exposure to microbes being a method of protecting against asthma.
But what the research hasn’t narrowed down is what type or level of exposure to dirt is good for you, although more recent studies are starting to provide some clues.
On a farm or in a household with pets, it could be microbes carried in dirt or bugs from the animals themselves, or it could be exposure to the proteins in animal fur.
A recent Finnish study found ‘farm-like’ microbes in dust (especially from soil) may be involved, even in urban homes.
A London School of Hygiene and Tropical Medicine study in 2004 found evidence that changing exposure to microbes could be a factor in the rise in allergies, but found no evidence that modern cleaning habits were to blame.
However, Otago University senior research fellow Caroline Shorter says research has come a long way since 2004. Having a very clean house may remove a lot of the ‘good’ microbes from dirt or dust that researchers are focusing on.
Hancox says one thing is for certain: a damp home is always bad, and children gain no benefit from growing up around mould, and are likely to suffer from it.
New Zealand’s housing is notoriously damp, which could be the reason for our high levels of asthma, rather than whether we’re exposed to dirt or not.
Hancox says parents should expose their kids to outdoor play, dirt, and animals.
The use of antibacterial sprays isn’t needed in a day-to-day setting, because they may kill the germs we have evolved to be around. While there’s no clear evidence that cleaning products cause asthma, they can trigger attacks in people who are already asthmatic.
University of Auckland pediatrics associate professor Cass Byrnes says cleaners are mainly a risk for triggering an attack when they are sprayed and can be breathed in, and not when they are sitting on a surface.
In other words, asthma sufferers are only likely to have an attack triggered if they are there when a cleaning agent is first opened, applied, or if the cleaner is sprayed into the air.
If people in your household suffer from asthma, it’s a good idea to consider if you need to use aerosol cleaners, and how and when you use them.
If you do continue using aerosol cleaners, it’s best to keep people with asthma or respiratory illnesses out of the room until the cleaner has been fully applied, or has a chance to settle.
Reporting disclosure statement: This post was written with expert advice from Otago University respiratory specialist Bob Hancox and University of Auckland pediatrics associate professor Cass Byrnes. This post was also reviewed by Otago University senior research fellow Dr Caroline Shorter.