HAVE WE BECOME TOO CLEAN FOR OUR OWN GOOD?

I sometimes think about how much has changed since then. Not just technology, education, family life, or society's attitude toward safety, but something as fundamental as our relationship with the biological environment. We have become extraordinarily good at protecting ourselves. Our homes are better insulated, our drinking water is monitored, our food is handled more safely, and we can fight infections that once claimed countless lives. These are enormous achievements. But what if some of the changes that have made life safer have also affected our health in ways we do not yet fully understand?

This is where a scientific theory with an almost provocative name enters the picture: the Hygiene Hypothesis.

When Dirt Became a Scientific Question

In 1989, British epidemiologist David Strachan published a study that would change the way researchers thought about allergies. He had investigated the relationship between family size and hay fever and discovered something unusual. Children who grew up with several older siblings generally had a lower prevalence of hay fever than children from smaller families.

Why should the number of siblings influence the risk of allergies? One possible explanation was that children in larger families were exposed to more microorganisms and infections early in life. Their immune systems therefore experienced different conditions during a period when they were still developing.

This became the starting point for the hygiene hypothesis. The original idea was that reduced exposure to certain infections during childhood might contribute to the increasing prevalence of allergic diseases.

Over the following decades, the research became considerably more complex, but also much more interesting. Scientists began to realize that the issue was not necessarily a lack of infections. Instead, attention shifted toward the enormous numbers of microorganisms with which humans have always lived.

We are not made up exclusively of human cells. Our bodies are home to extensive communities of bacteria, fungi, viruses, and other microorganisms. They live on our skin, in our mouths, and especially in our intestines. Together, these communities form our microbiome, which plays an important role in digestion, metabolism, and the development of the immune system.

Human beings are therefore not isolated biological individuals. We are also ecosystems.

The Immune System Is More Than a Defense Army

We have long described the immune system as an army fighting invaders. It is a useful image, but an incomplete one. The immune system must not only recognize and combat dangerous microorganisms. It must also learn to tolerate everything that is not dangerous.

Pollen is normally not a threat. Peanuts are food. Animal dander is not an enemy. Yet in some people, the immune system reacts as though these substances were potentially life-threatening.

An allergy is, in a sense, a biological misunderstanding. The body's defenses react to something that would not normally justify such a response.

During childhood, the complex mechanisms that help the immune system distinguish between threats and harmless substances are still developing. Exposure to microorganisms plays a role in this process. Research suggests that a diverse microbial environment can contribute to the development of immunological tolerance, meaning the body's ability to respond proportionately.

This does not mean that children need to become sick to develop a healthy immune system. On the contrary, serious infections can damage health. The important distinction is between exposure to disease-causing pathogens and living in contact with the microbial diversity alongside which humans have evolved.

And that distinction is essential.

What Did the Farm Children Teach Us?

One of the most fascinating studies was published in 2016 in the medical journal The New England Journal of Medicine. Researchers compared children from two North American agricultural communities: the Amish and the Hutterites.

The groups share some European ancestry and several cultural traditions, but their farming environments differ. Amish families often practice traditional agriculture, where children live close to barns, animals, and everyday farm activities. The Hutterites make greater use of mechanized farming methods.

The findings were remarkable. Among the children studied, approximately five percent of the Amish children had asthma, compared with more than twenty percent of the Hutterite children. Allergic sensitization was also considerably less common among the Amish. Researchers additionally found much higher concentrations of bacterial endotoxins in airborne dust from Amish homes.

There were clear differences in the children's immune systems, and supplementary laboratory experiments supported the possibility that certain environmental exposures could contribute to protective immune responses.

This was not proof that dirt in general is healthy. Endotoxins and agricultural dust can also be harmful. Nor did the study identify a single bacterium responsible for the results. But it demonstrated how strongly different childhood environments can be associated with immune development.

This raises an important question: Have we changed children's microbial surroundings so rapidly that we have also changed the biological conditions under which they develop?

From Garden Hoses to Hand Sanitizer

A video from the YouTube channel Growing Up Unseen describes six typical behaviors among children in the 1970s. We drank from garden hoses, ate food that had fallen on the floor, shared bottles, allowed dogs to lick us, played in the dirt, and did not constantly disinfect our hands.

It is easy to recognize those memories. But it is equally easy to draw conclusions that go too far.

Drinking water from an old garden hose has not been shown to prevent allergies. Sharing bottles can spread infections. Nor is there scientific evidence that children become healthier by eating food from dirty floors. The so-called five-second rule is not a reliable food safety principle.

However, research does link early exposure to animals and certain natural environments with a lower risk of allergic diseases. A major Swedish registry study published in 2015, for example, found that children who lived with a dog during their first year had lower odds of asthma at age six. This was an observed association, not proof that dogs prevent asthma in every child.

We therefore do not need to romanticize dirt to recognize that our surroundings matter.

The truly interesting point is not that we drank from garden hoses. It is that we may have spent more time in environments where we naturally encountered microorganisms different from those that dominate our modern indoor spaces.

Peanut Allergies Changed Medical Thinking

Another important scientific breakthrough came in 2015 with the LEAP study. For many years, concerns about allergenic foods had led to recommendations to delay the introduction of foods such as peanuts for certain children.

But the researchers behind LEAP tested a different approach. Infants at increased risk of peanut allergy were introduced to peanut products early, under controlled conditions. A comparison group avoided peanuts.

By the age of five, peanut allergy was significantly less common among children who had consumed peanuts early. In the central analysis group, the prevalence was 3.2 percent compared with 17.2 percent among children who had avoided peanuts. This represented approximately an 81 percent relative reduction in risk.

The study transformed recommendations concerning the early introduction of allergenic foods. It demonstrated how the immune system can develop tolerance through early exposure to a specific food. It did not, however, mean that random exposure to dirt produces the same effect. Children with suspected peanut allergy or a high risk of developing it may require medical assessment before introduction.

What makes this especially interesting is how scientific research sometimes forces us to reconsider ideas we have long taken for granted.

Modern Childhood Is a Social Experiment

At this point, I want to move beyond the strictly medical discussion for a moment.

When I look at the changes since my own childhood, I see a society that has gradually attempted to eliminate uncertainty. We schedule activities, monitor children's movements, measure health, control food, and build increasingly sophisticated homes.

At the same time, many children spend much of their daily lives indoors. Urbanization, traffic, digital entertainment, and changing family habits influence what childhood looks like. That does not mean every child enjoyed greater freedom in the past or that childhood today is necessarily worse. But the environments in which children grow up have changed in several fundamental ways.

We have developed technology and social systems at a speed unprecedented in human history. Biological evolution, however, operates across much longer timescales.

This does not mean that the human body is incapable of adapting to new environments. But there is every reason to investigate what happens when our lifestyles change faster than our understanding of their consequences.

The hygiene hypothesis then becomes part of a much larger story. A story about humanity's relationship with nature, the unintended consequences of modernization, and the strange assumption that we can improve our lives by isolating ourselves from everything we cannot completely control.

When Control Becomes an End in Itself

Human beings have a tendency to regard whatever we can measure, regulate, and eliminate as progress. If bacteria can cause disease, fewer bacteria must be better. If an activity involves risk, it must be restricted. If something is unknown, it must be controlled.

But biological systems rarely operate so simply.

Not all bacteria are dangerous. Not all inflammation is harmful. And an immune system that reacts aggressively to everything is not necessarily a better immune system.

A well-functioning biological system often depends on balance and regulation, rather than eliminating every possible influence.

At the same time, we must be absolutely clear about one thing. Clean drinking water, sanitation systems, vaccinations, antibiotics, and good food hygiene have saved an enormous number of lives. There is no reason to return to an era when children were exposed to infections that can now be prevented.

Modern research is therefore not about abandoning hygiene. It is about understanding the difference between preventing infection and preserving a healthy biological environment.

The term Old Friends Hypothesis describes this more accurately than the original hygiene hypothesis. The idea is that humans evolved in close interaction with microorganisms that help regulate our immune systems. When our exposure to parts of this microbial world changes, our immunological development may also be affected.

But this remains an area of research with many unanswered questions. Allergies are also influenced by genetics, environmental factors, skin barrier function, diet, and several other mechanisms. No single theory explains the entire development.

The Society of the Future Must Understand Humans Better

I believe one of the most important conclusions is that we need to start viewing our cities, homes, and living environments as biological systems, not merely technological constructions.

Today, we build intelligent houses that can detect when people are present in a room. We can automate ventilation, heating, lighting, and security. But how much do we really think about the microbial environment in which people spend their lives?

Could future urban planning contribute to better health through more green spaces, natural play environments, and richer biodiversity? Could schools and residential neighborhoods be designed to give children greater opportunities to interact with nature without exposing them to unnecessary dangers?

These are questions that deserve research and experimentation, not quick commercial solutions. We still do not know exactly how buildings could be optimized to improve human microbial exposure safely and measurably.

But we know enough to question the assumption that human environments should be designed solely for efficiency, control, and the greatest possible biological separation from the natural world.

Were Children in the 1970s Healthier?

The answer is not as simple as nostalgic videos sometimes suggest.

We cannot conclude that children in the 1970s were generally healthier. There is insufficient directly comparable historical data for several allergic conditions, and in many other areas of health, children today have significantly better prospects. We also diagnose diseases differently than we did in the past.

But there are sound scientific reasons to take the question of changing microbial environments seriously.

It is entirely possible that urbanization, changing lifestyles, and other aspects of modernization have affected the environments in which our immune systems develop. Research involving farm children, the microbiome, and early allergen exposure demonstrates that these relationships matter, even if the mechanisms are not yet fully understood.

When I look back on my childhood, it is not because I want to return to contaminated water pipes, infections, or inferior healthcare. It is about something much more fundamental.

We lived closer to our surroundings. We spent time outdoors, moved around, discovered things, found our own way, and learned through direct experience. That did not automatically give us stronger immune systems or better lives. But it represented a different kind of everyday contact with the world.

Perhaps it is precisely that contact we need to understand better.

Because the real lesson of the hygiene hypothesis is not that dirt makes us strong. It is that humanity has never evolved independently of its environment. Our bodies, our brains, and our immune systems are the products of an extraordinarily long interaction with the living world.

In just a few generations, we have changed almost everything around us. Yet we still do not fully understand what all these changes are doing to us.

And perhaps that is where one of the greatest challenges facing modern humanity lies: not in developing even greater control over our surroundings, but in learning which parts of them we actually need to live alongside.

Scientific References

• Strachan, D. P. (1989). Hay fever, hygiene, and household size. British Medical Journal. DOI: 10.1136/bmj.299.6710.1259

• Stein, M. M. et al. (2016). Innate Immunity and Asthma Risk in Amish and Hutterite Farm Children. New England Journal of Medicine. DOI: 10.1056/NEJMoa1508749

• Fall, T. et al. (2015). Early Exposure to Dogs and Farm Animals and the Risk of Childhood Asthma. JAMA Pediatrics. DOI: 10.1001/jamapediatrics.2015.3219

• Du Toit, G. et al. (2015). Randomized Trial of Peanut Consumption in Infants at Risk for Peanut Allergy. New England Journal of Medicine. DOI: 10.1056/NEJMoa1414850

• Gupta, R. S. et al. (2018). Prevalence and Severity of Food Allergies Among US Children. Pediatrics. DOI: 10.1542/peds.2018-1235

Video reference: Growing Up Unseen – 70s Kids and the Hygiene Hypothesis