Good clean muck
Here in New Zealand one in three of us will develop an allergy at some time in our lives. Is our obsession for cleanliness behind all our wheezing, sneezing and itching? And would our bodies benefit from a bit more dirt?
Spring is here and any day now I expect my eyes will start itching and my nose running: I had asthma as a child and I’m still susceptible to a good wheeze when a fluffy cat or feather duvet comes my way.
This is nothing unusual. One in four New Zealand children will develop asthma and 30 per cent of the western world suffers from allergies.
It’s still largely a mystery as to why allergies, and in particular allergic asthma (extrinsic – the most common form) develop, but one theory that’s been knocking around since the late 80s is the hygiene hypothesis.
The basic tenet of this hypothesis is that exposure to pathogens, bacteria, dirt and dust in some way prevents groups like farmers from developing allergies and asthma.
Meanwhile the urban dwellers with their more sanitised environments have lost exposure to the things that were keeping allergies at bay.
Massey University scientists are testing the fact behind the theory with series of studies trying to isolate the factors that make farming families in New Zealand’s lower North Island less susceptible to asthma, hay fever and eczema.
Professor Jeroen Douwes, epidemiologist and director of Massey University’s Centre for Public Health Research, is the lead investigator of this study and says for him it provides strong evidence of support for the hygiene hypothesis.
“I think there’s nothing better to replace the hygiene hypothesis at the moment. If you look at the most recent studies it does confirm that people exposed to environments with microorganisms appear to have a lower prevalence of asthma.
“There are a number of inconsistencies but I think it’s still a good working hypothesis as it explains the reduced existence of allergies in farmers’ children.”
However he says the hypothesis doesn’t explain the differences in allergy levels between certain countries.
“If you compare Brasil and South American countries with Spain and Portugal, the South American countries have higher prevalence of asthma even though they are generally seen to be ‘dirtier’ than Spain and Portugal.
“In many western countries we now see a decline in asthma prevalence where for many years we’ve seen an increase. If the hygiene hypothesis was the main driver of that decline it would appear people are now less clean and are getting more exposure to the microorganisms, which is unlikely to be the case.”
He says some of the bugs we encounter in daily life can cause infections and have consequences more serious than asthma but it’s important to find a balance between being filthy and highly sanitised.
“I think we can probably be more relaxed with children playing in the dirt. We don’t need to be completely paranoid about making sure children are not exposed to anything. We shouldn’t be concerned about them getting dirty and in fact a little bit of exposure to microorganisms is likely to be beneficial.
“There are two types of exposure; one is protective and one is more likely to make you susceptible. For example exposure to diesel exhaust emissions may make you more susceptible and exposure to microorganisms or dirt may reduce that risk.
He says studies suggest it’s important to have both early and life-long exposure to protective factors to keep allergies at bay.
“If you were thinking about protecting yourself as well as you could you’d prefer to be exposed as early as possible, which could mean in utero.”
However he stresses one hypothesis won’t explain allergies away and there’s still a lot of mystery surrounding them.