Is being too clean making kids sick (dirt, farms, siblings, pets)?

Where the idea came from

In 1989, a BMJ paper noted that hay fever was less common in children from larger families, especially those with more older siblings. Its author suggested that infections passed around in early childhood might protect against allergy⁷. The pattern has held up in far larger later studies, mainly in wealthier countries, although severe asthma and eczema were more common in larger families⁸. The idea was soon shortened to the "hygiene hypothesis."

Why researchers want a new name

Clean homes are probably not the culprit. A 2016 panel of six experts found no good evidence that hygiene as the public understands it, such as washing hands or cleaning kitchens, is responsible for the changes in microbial exposure behind the rise in allergy¹. More likely suspects are urban living, less time outdoors, changes in diet and antibiotic use, all of which change the microbes we meet and carry¹.

The related "old friends" idea holds that people evolved alongside microbes from soil, animals, water and other people, and that the immune system relies on them to learn when not to react. With less of that input, immune control may weaken, showing up as allergy or chronic low-level inflammation⁹.

What farms and pets show

Children who grow up on traditional farms, with cows, straw and farm milk, have less asthma, hay fever and eczema than other rural children³. Their bedroom dust held a wider variety of microbes, and that variety explains a good share of the difference². In the United States, Amish children had far less asthma than Hutterite children despite similar genes and lifestyles, and the two groups farm differently¹⁰.

Pets are less clear. Swedish and Finnish studies found less asthma in children who lived with a dog in infancy^11,12, but a pooled analysis of 11 European birth cohorts found no overall link between early pets and asthma⁵.

What still protects us

Washing hands with soap cut diarrhea risk by an estimated 42-47% in a pooled analysis of 17 studies⁴. The expert panel's answer is "targeted hygiene": clean where it stops infection, such as hands, food, toilets and wounds, and relax about everyday dirt¹. For the microbe side of the story, see Dirt, Soil Microbes, and Immune Health.

What to take from this

These studies show patterns across groups of children, not what will protect your own child. If a child has eczema, wheeze or a possible food allergy, a pediatrician's advice comes first; for preventing food allergy in babies, see Early Introduction of Peanut and Egg.

Farm exposure: two large cross-sectional studies

The GABRIEL Advanced Studies screened 79,888 school-aged children in rural Austria, Germany and Switzerland, then collected detailed questionnaires from 8,419 and allergy antibody results from 7,682³. Children living on a farm had lower odds of asthma (adjusted odds ratio 0.68, 95% CI 0.59-0.78), hay fever (0.43, 0.36-0.52), atopic dermatitis, or eczema (0.80, 0.69-0.93), and allergic sensitization, meaning allergy antibodies in the blood (0.54, 0.48-0.61), than non-farm children³. Contact with cows, straw and farm milk accounted for the asthma effect, but no measured exposure fully explained the effect on hay fever and sensitization³. Limitations: parents reported the exposures, and farming families may differ from others in ways no one measured.

A companion analysis asked which microbes might be responsible². In two cross-sectional studies, PARSIFAL (bacterial DNA in mattress dust) and GABRIELA (cultured bacteria and fungi in settled dust), farm children met a greater variety of microbes, and greater variety went with less asthma (odds ratio 0.62, 95% CI 0.44-0.89, in PARSIFAL; 0.86, 0.75-0.99, in GABRIELA)². The authors concluded that microbial variety explained a substantial fraction of the farm effect². Because exposure and disease were measured at the same time, cause and effect cannot be separated.

Siblings and family size

The International Study of Asthma and Allergies in Childhood analyzed questionnaires from 210,200 children aged 6-7 in 31 countries and 337,226 aged 13-14 in 52 countries⁸. Reported hay fever and eczema fell as the number of siblings, and especially older siblings, rose (P<0.0001), and the inverse pattern was significantly stronger in more affluent countries⁸. By contrast, symptoms of severe asthma and severe eczema rose with family size in both age groups⁸. The study is questionnaire-based and cross-sectional, so it shows patterns, not mechanisms.

Same genes, different farming

Stein and colleagues compared 60 Amish and Hutterite children in the US¹⁰. Asthma and allergic sensitization were 4 and 6 times less common in the Amish, and house-dust endotoxin, a bacterial component that stimulates innate immunity, was 6.8 times higher in Amish homes¹⁰. Dust extract from Amish but not Hutterite homes protected mice against allergic airway disease, and the protection depended on two innate immune signaling molecules, MyD88 and Trif¹⁰. Limitations: only 60 children from two communities, and mouse findings do not prove the same mechanism in people.

Pets and early animal contact

In a Swedish register study of all 1,011,051 children born in 2001-2010, living with a dog in the first year of life was linked to less asthma at age 6 (odds ratio 0.87, 95% CI 0.81-0.93), and farm animal contact to a larger reduction (0.48, 0.31-0.76)¹¹. In preschoolers the dog link appeared at age 3 and older (hazard ratio 0.90, 0.83-0.99) but not under 3 (1.03, 1.00-1.07)¹¹. A Finnish cohort of 3,781 children found less asthma by age 5 with a dog at home (hazard ratio 0.60, 0.38-0.96) and less eczema with a cat (odds ratio 0.68, 0.51-0.92), while farm animals showed no link¹². Both studies are observational. Families with allergic children may avoid pets, which could produce part of the pattern without any protection.

Conflicting results

The broadest pet analysis pooled individual data from 11 European birth cohorts that recruited more than 22,000 children in the 1990s⁵. Early cat ownership showed no link to asthma at age 6-10 (odds ratio 1.00, 95% CI 0.78-1.28; 10 studies, 11,489 children), and the dog result was not statistically significant (0.77, 0.58-1.03; 9 studies, 11,433 children)⁵. Furry pets in the first two years were linked to less sensitization to airborne allergens, but the authors concluded that clinicians should not advise getting or avoiding pets to prevent asthma or allergic rhinitis⁵. Differences in timing, pet type and era may explain why registry cohorts and pooled birth cohorts disagree, but the disagreement means the pet effect is not settled.

Hygiene and infection

Curtis and Cairncross pooled 7 intervention studies, 6 case-control, 2 cross-sectional and 2 cohort studies⁴. In the trials, not washing hands carried a relative risk of diarrhea of 1.88 (95% CI 1.31-2.68), which implies that handwashing with soap could cut risk by about 47%; pooling all studies, only high-quality studies, or only studies that mentioned soap gave reductions of 42-44%⁴. The authors warn that many studies were of poor quality and that publication bias may inflate the result⁴. On raw milk, a US outbreak model estimated that unpasteurized dairy products cause 840 times more illnesses and 45 times more hospitalizations than pasteurized ones⁶.

What the evidence cannot show

None of the studies above randomly assigned children to more or fewer microbes. They show patterns across populations, not that adding dirt, a dog or a farm visit prevents allergy in a particular child, and a mouse experiment cannot stand in for a human trial.

Bottom line (confidence 3 of 5)

Farm living and wide microbial variety are consistently linked with less childhood asthma and allergy, so the "old friends" idea is plausible. But the evidence is observational, the pet results conflict, and "too clean" is probably the wrong explanation. Keep hand and food hygiene, and treat dirt, pets and farms as possible extras rather than prevention strategies.