The hygiene hypothesis proposes an explanation about the development of immune disorders. This includes asthma, an inflammatory airway disease that involves an abnormal immune response.

According to the theory, people who grow up in extra clean, modern environments aren’t exposed to normal levels of infectious pathogens. This prevents proper development of the immune system.

The theory is based on the fact that these pathogens can stimulate the immune system. They trigger cellular pathways that regulate immunity, thus affecting immune response.

There’s some evidence to the hygiene hypothesis. It’s supported by the high rate of immune disorders in countries with higher levels of sanitation.

However, it’s a generalized explanation, as it doesn’t tell the full picture of immune disorders. According to research, other factors unrelated to hygiene also contribute to such conditions.

Read on to learn about the hygiene hypothesis, including its history and limitations.

The hygiene hypothesis is based on the concept that pathogens stimulate the immune system. It’s been explained by the relationship between pathogens and T helper cells, according to a 2017 research review. T cells are white blood cells in the immune system.

Essentially, infectious pathogens typically activate T helper 1 (Th1) cells. This causes cytokines to be released, which are signaling proteins that regulate the immune system. In contrast, reduced exposure to infectious germs decreases Th1 activity. This causes the body to compensate by boosting T helper 2 (Th2) cell activity.

High Th2 activity is characteristic of allergic disorders. This includes allergic asthma, or asthma triggered by certain substances, according to a 2021 research review.

Thus, it was thought that high Th2 activity due to reduced exposure to infectious pathogens contributes to allergic disorders like asthma. This hypothesis was widely accepted by the medical community and public.

Today, however, the hypothesis is less accepted. It’s considered to be an oversimplified explanation. After all, allergic and other immune disorders are complex conditions, and the hygiene hypothesis only highlights one possible cause.

What about the hygiene hypothesis for food allergies?

Researchers have also studied the hygiene hypothesis for food allergies. A food allergy occurs when the immune system overreacts to a food substance.

According to another 2016 research review, reduced microbial exposures during childhood can contribute to food allergies. But other nonhygiene factors, like low vitamin D levels and timing of food introductions, also appear to increase the risk.

A 2016 study also found a link between food allergies and hygiene, including having many siblings or attending child care centers. But the study found an even stronger association between food allergies and eczema, as well as skin infections.

This is worth noting because food allergies may be rooted in skin exposure to allergens. Eczema and skin infections can exacerbate this, as both conditions weaken the skin barrier. This suggests skin barrier function, and not just hygiene, contributes to food allergies.

The hygiene hypothesis was first proposed by David Strachan, an epidemiologist in London, in 1989. He developed the theory after completing a study that year about hygiene.

The study included more than 17,000 British children. According to Strachan’s findings, babies who grew up in a home with multiple siblings were less likely to develop eczema before they turned 12 months old. They were also less likely to develop hay fever, or allergic rhinitis, later on.

Strachan proposed that early exposure to infectious diseases can provide protection against allergies. This was based on the idea that a home with many children had more pathogens.

These findings became widely accepted by the medical community and public. Eventually, it was studied as an explanation for many immune disorders, including asthma.

In 2000, Strachan named this concept the “hygiene hypothesis.”

Over the last few decades, researchers have found some contradictions in the hypothesis.

One issue involves helminth infections, which are caused by parasitic worms transmitted through the soil. They’re rare in industrialized nations, where living environments are generally cleaner, and common in developing countries.

Helminth infections are associated with lower rates of immune disorders, including asthma. In fact, according to another 2021 research review, helminth infections may protect against asthma. That’s because the parasite hinders the body’s immune response to stay alive, according to a 2015 research review. This reduces the overactive immune response usually seen in asthma.

However, helminth infections are marked by increased Th2 activity, according to a 2018 research review. This contradicts the hygiene hypothesis.

Another issue is that some pathogens actually increase the risk of asthma. For example, a different 2021 research review found that the virus Chlamydia pneumoniae, which causes pneumonia, can trigger or worsen asthma. A 2021 study found that respiratory syncytial virus (RSV), which causes bronchiolitis, increases the chance of asthma in children.

Though exposure to some pathogens does stimulate the immune system, other pathogens can have a negative effect. Also, the human microbiome (the microbes in and on us) have a significant role in immunity. This microbiome is regulated by many factors, like diet and antibiotic overuse, which are unrelated to hygiene.

Asthma often runs in families, meaning genetics play a role. But it may be possible to prevent or delay the development of asthma in some children.

The following strategies may help:

  • Breastfeeding. Breastfeeding might reduce the chance of asthma. According to a 2018 research review, human milk contains beneficial compounds that may improve lung development and immunity in infants.
  • Reduce exposure to dust mites. According to a 2020 research review, exposure to dust mites can contribute to asthma. You can minimize exposure by limiting carpets, frequently washing bedding, and using impermeable protective coverings on mattresses and pillows.
  • Reduce exposure to secondhand smoke. Exposure to tobacco smoke is a risk factor for childhood asthma, according to a 2021 research review.

Your child’s doctor can offer additional strategies based on your family history and lifestyle.

According to the hygiene hypothesis, immune disorders like asthma are due to excessively clean environments. It states that these conditions reduce exposure to infectious pathogens, which would typically stimulate healthy immunity.

It’s true that some pathogens can benefit the immune response. But research has found that certain pathogens can actually trigger asthma. Additionally, not all factors that contribute to asthma are related to hygiene.

In general, the hygiene hypothesis is considered to be oversimplified. Asthma and other immune disorders are due to a range of factors, which are continuously being studied by researchers.