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?

Element - - Global Health - By So­phie Bond

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.

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