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On Household Molds and the Hygiene Hypothesis

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The Hygiene Hypothesis has been making the rounds for decades now and is recently gaining momentum, alongside the growing public interest on the human gut microbiome. Proponents of the hypothesis are convinced that the increasing incidence of allergies, particularly in developed countries, has to do with modern society's obsession with cleanliness — hello hand sanitizers!

The lack of exposure to fungi and bacteria early in life, according to the hypothesis, deprives the immune system of its right to build a memory of such microbial invaders and wage a war against them if they are present in large numbers. The end result is a somewhat unreliable immune system, manifested via autoimmune disorders and frequent occurrence of the common cold, flu, and asthma attacks.

While the Hygiene Hypothesis remains a hypothesis, it is also interesting to note that several studies have shown that children living in farms or whose mothers had farm exposure during pregnancy were less likely to have asthma and other allergic reactions later in life. Also known as the Farm Effect, this phenomenon has been extensively reviewed in scientific literature.

Not All Microbes Are Created Equal

As mentioned earlier, recent findings on the human gut microbiome have made it clear that microbes, may it be fungi, virus or bacteria, play a large complex role in helping us achieve optimum health. Although we are now aware that microbes are not merely foes but are actually beneficial, the Hygiene Hypothesis should not be an excuse for us to slack off in our fight against household molds.

Health Issues Resulting From Household Molds

Notably, several health conditions and issues are strongly linked to the occurrence of household molds.


In 2014, researchers from the University of Exeter in the United Kingdom conducted a comprehensive review of 17 clinical studies associating household molds and asthma.

Molds are abundant in our outdoor and indoor environments, with around 10 varieties living in a typical home. We’ve found the strongest evidence yet of their potentially harmful effects, with higher levels of some of these molds presenting a breathing hazard to people suffering from asthma, worsening their symptoms significantly,”  Richard Sharpe, head researcher of the study emphasized.


Meanwhile, a large-scale study headed by Brown University epidemiologist Edmond Shenassa was originally initiated to disprove the connection between depression and indoor molds which earlier studies suggest. However, Shenassa and his team were surprised when they found solid evidence linking indoor molds and mental health issues.


Here's an excerpt of the ScienceDaily post about the study:

Shenassa noted the study, an analysis of data from nearly 6,000 European adults, does not prove that moldy homes cause depression. The study wasn't designed to draw that direct conclusion. However, Shenassa's team did find a connection, one likely driven by two factors. One factor is a perceived lack of control over the housing environment. The other is mold-related health problems such as wheezing, fatigue and a cold or throat illness.

Aside from asthma and depression, other health issues associated with indoor molds include miscarriage, severe PMS (premenstrual syndrome) symptoms, sinus infections, and multiple sclerosis.

According to the Centers for Disease Control and Prevention (CDC), indoor mold infestation does not always present a health problem. However, certain mold species can potentially lead to serious health issues. Although mold sampling can help identify which fungal strains are present, the CDC recommends taking measures to prevent and remove molds instead of identifying the strains.

Since the susceptibility of individuals can vary greatly either because of the amount or type of mold, sampling and culturing are not reliable in determining your health risk. If you are susceptible to mold and mold is seen or smelled, there is a potential health risk; therefore, no matter what type of mold is present, you should arrange for its removal, the CDC recommends.


Upgrading air filters in your HVAC system and changing filter sizes is one good way to improve indoor air quality and reduce mold infestation at home. Additionally, improving AC filter sizes will not only reduce mold spores but they can also significantly lead to a decline of other potential allergens such as pollen and dust.