There are news about the hygiene hypothesis.
Home cleanliness resulted only in quantitative reduction of floor dust, which mainly indicates removal of superficial dirt with a rather cosmetic effect. Conventional cleaning does not eradicate microorganisms sustainably, because emptied microbial niches are instantly recolonized by ventilation and living carrier.
In the present study population, however, there were clear inverse associations of endotoxin concentration in the first year of life with asthma and atopic sensitization, mattress dust weight and endotoxin load with eczema and hay fever, mattress muramic acid with eczema, and floor muramic acid with asthma (Figure 3).
Not sure, if I look at figure 3:
Because we found inverse associations of cleanliness with the dust parameters and inverse associations of dust parameters with asthma and allergies, we expected positive associations of cleanliness with the disease outcomes. The expectation, however, was not met.
I never expected any association. And this was a realistic expectation.