Dear Doctor: Some researchers say the main reason for many health issues today, including allergies and asthma, is that our children are kept too clean and have no resistance to anything. What are your thoughts?
Dear Reader: First, let's discuss the benefits of hygiene. The greatest boon to human longevity, at least in developed parts of the world, has been our ability to deal with wastewater. Separating humans from the waste they produce, and treating that waste, has decreased our susceptibility to a host of diseases. In Yemen, for example, the number of cases of cholera currently exceeds 800,000 due to the inability to manage wastewater. In fact, inadequate sanitation, combined with a limited ability to properly disinfect hands, is estimated to cause 4 percent of deaths worldwide.
But yes, there may be a risk to too much good hygiene. Humans live in symbiosis with the bacteria within and upon their bodies, and an inherent balance is necessary for a healthy life. This balance may extend to the bacteria in our environment, meaning that restricting our bodies from normal outdoor organisms may have a downside. The Amish in the United States, for example, live on farms and spend much of their time outside. Coincidentally, their children have much lower rates of allergies, eczema and asthma compared to non-Amish children who don't live on farms.
The evidence extends beyond that correlation. A 2017 study of adult farmers and their spouses compared their childhood exposure to a farm environment against their rates of allergies and asthma. Those who were exposed to farming as children were less likely to have allergies as adults. Further, those whose mothers worked on a farm while pregnant also had a lower rate of asthma and allergies.
Then there was a Swedish study on pacifier cleaning. Researchers found that the parental practice of cleaning a pacifier by putting it in one's own mouth before giving it back to the infant was linked to a lower rate of eczema and asthma in children at age 18 months. That same study also showed that vaginal birth, with its increased infant exposure to bacteria, was linked to a lower rate of allergies at 18 months compared to Caesarean delivery.
Other studies have shown that having an older sibling decreases a child's risk of allergies and eczema, supposedly because older siblings bring more bacteria into the house. Similarly, having a pet in early life (especially if it's a dog) has been linked to a lower rate of allergies and asthma in children.
But bacteria may just be one factor in the allergy equation. Exposure to allergens themselves may desensitize a child to them later in life. That brings us to the fact that children and their parents overall are spending less time outside. At the same time, children in densely populated urban areas are exposed to pollutants at younger ages. Both factors may increase the risk of allergies and asthma.
In summary, it's clear that the rates of asthma and allergies have incrementally increased in the United States over the last 50 years. Although the research is largely correlative, it seems equally clear to me that we can reduce that risk in our children by getting them -- and ourselves -- outside the house as much as possible, exposing them to their surrounding biome (within reason), and pushing for a reduction in pollution.
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