Good hygiene doesn’t damage the immune system in children

Keeping our homes clean may not be bad for our immune system after all, defend researchers at University College London and the London School of Hygiene & Tropical Medicine, UK (1). According to a study published in the Journal of Allergy and Clinical Immunology, people should live in a home environment free of germs. The team defends that we need contact with microbes to build up our immune system, but this should be done outside in touch with nature.

The “hygiene theory”, first developed in the 1980s, argues that living in an almost sterile environment at home means children lack exposure to microbes and are more likely to develop certain conditions like allergies.

Now, researchers from University College London and the London School of Hygiene & Tropical Medicine suggest that the connection between hygiene and developing allergies is not as straightforward as it once seemed. The team argue that good hygiene at home should not be ignored for the sake of maximising our immune system. Kids need to be exposed to microbes, but it needs to be the right type of microbes.

“Exposure to microorganisms in early life is essential for the education of the immune and metabolic systems,” said Prof Graham Rook, lead author in this study. “But for more than 20 years, there has been a public narrative that hand and domestic hygiene practices, that are essential for stopping exposure to disease-causing pathogens, are also blocking exposure to the beneficial organisms.”

The team showed that good bacteria are usually introduced to children very early in their lives, coming from family members and not items at home. In contrast, there’s very little evidence to suggest that nasty bacteria typically found in a home are vital for developing a healthy and balanced immune system. Also, kids get exposed to plenty of germs through routine vaccinations during childhood. This “trains” their immune system in the same way as actually being sick, in most cases without any symptoms.

However, there is one way where excessive cleaning may be harmful. The team believes exposure to strong chemicals found in cleaning products can cause many immune conditions, such as allergies and asthma.

For the authors, the purpose of this study is not to convince people that the “hygiene theory” is totally wrong but to think about it in a more nuanced way. Rather than aim to live in a sterilised environment with potential damage caused by exposure to chemicals, we should think about targeted hygiene. For example, if we’re about to eat something, we should wash our hands first, but there’s no need to deep clean our homes before every meal.

“So cleaning the home is good, and personal cleanliness is good, but, as explained in some detail in the paper, to prevent the spread of infection, it needs to be targeted to hands and surfaces most often involved in infection transmission. By targeting our cleaning practices, we also limit direct exposure of children to cleaning agents,” Rook said. “Exposure to our mothers, family members, the natural environment, and vaccines can provide all the microbial inputs that we need. These exposures are not in conflict with intelligently targeted hygiene or cleaning.”

(1) Rook G and Bloomfield S (2021) Microbial exposures that establish immunoregulation are compatible with targeted hygiene. The Journal of Allergy and Clinical Immunology, 148: 33-39, https://doi.org/10.1016/j.jaci.2021.05.008