I have long been interested in the phenomenon of hormesis and teach this topic to environmental economics students. This idea – developed by toxicologists – suggests that low concentrations of certain apparently dangerous substances (gamma rays, dioxins, even pesticides), may be good for your health. The health damage function is therefore J or U-shaped in levels of the toxin. Damages are high at zero concentrations of certain compounds and high (indeed higher) at high concentrations. But at small enough concentrations the damages are lower so that it is best from a health perspective if you are exposed to some of the compounds.
My intuitive reasons for being interested in this link is that kids seem increasingly vulnerable to allergies and asthma even though mum washes them and everything around them to be as spotlessly clean as Kevin Rudd's ear canals.
After a recent class where I talked about hormesis one of my students put me onto the related idea in a recent issue of New Scientist of ‘Fighting Cancer with Filth’. The gist of the article is that ‘researchers are starting to wonder whether the higher incidence of certain cancers in affluent populations - including breast cancer, lymphoma and melanoma - might also have something to do with sanitised, infection-free living’ so that ‘If we can understand exactly what it is about some germs that has a protective effect, we should be able to reduce people's risk of developing certain tumours later in life by exposing them to harmless microbes.’ ‘The simplest explanation is that exposure to viruses and bacteria - through infections, vaccinations or proteins such as endotoxin - stimulates the immune system and boosts its anticancer activity’. ‘As long ago as the 1970s it was noticed that workers in cotton factories have surprisingly low rates of lung and some other cancers. One explanation is that they owe the favour to cotton dust. This contains lots of endotoxin, a lipopolysaccharide found in the cell walls of many bacteria, which might keep the immune system on high alert.’ ‘The simplest explanation is that exposure to viruses and bacteria - through infections, vaccinations or proteins such as endotoxin - stimulates the immune system and boosts its anticancer activity. This makes sense, since we now know the immune system plays a bigger role in battling cancer than previously thought, destroying many tumours at an early stage or keeping them in check’.
There is even evidence that dairy workers who breathe in dried cow dung dust have a lower incidence of long cancer. There are even suggestions that injecting certain bacteria and viruses might cure cancer or even treat depression.
Hormesis arguments are hypotheses about behaviour but interesting ones. The implication of these ideas for the management of risk in relation to exposure to toxic substances, however, is huge. The dose-response relationship may need to be rethought in connection with the management of both dirt and toxins.
In addition the scorn conventional medicine has toward homeopathy may need to be rethought.