{"url":"https://whatshealthyandwhy.com/hygiene-hypothesis/too-clean-the-hygiene-hypothesis","path":"/hygiene-hypothesis/too-clean-the-hygiene-hypothesis","alternates":{"json":"https://whatshealthyandwhy.com/api/articles/hygiene-hypothesis/too-clean-the-hygiene-hypothesis.json","markdown":"https://whatshealthyandwhy.com/hygiene-hypothesis/too-clean-the-hygiene-hypothesis.md"},"topic":{"slug":"hygiene-hypothesis","name":"Hygiene Hypothesis","url":"https://whatshealthyandwhy.com/hygiene-hypothesis"},"slug":"too-clean-the-hygiene-hypothesis","order":1,"title":"Too Clean? The Hygiene Hypothesis","question":"Is being too clean making kids sick (dirt, farms, siblings, pets)?","answer":"Cleanliness itself is unlikely to be the cause, but less early contact with farms, animals and varied microbes is linked to more asthma and allergy.","description":"Why experts now say microbe-poor childhoods, not clean homes, may drive allergy and asthma, and what farms, pets, siblings and handwashing really do.","confidence":3,"confidenceNote":"Large studies agree that farm and microbe exposure tracks with less asthma and allergy, but these studies do not show that adding exposure prevents disease, and pet findings are mixed.","evidenceSummary":{"rctCount":0,"observationalCount":7,"metaAnalysisCount":2,"totalParticipants":"1,000,000+","replicated":true,"newestStudyYear":2020},"category":{"categoryTitle":"Your Body","categorySlug":"your-body","subcategoryName":"Family, Kids & Pets","subcategorySlug":"family","url":"https://whatshealthyandwhy.com/explore/your-body/family"},"actions":["Keep washing hands with soap before eating, after the toilet and after handling raw meat or animals; in a pooled analysis it cut diarrhea risk by an estimated 42-47% [4].","Give children time outdoors in parks, gardens and green spaces; outdoor activity and less indoor time are among the strategies experts think may help restore a healthy microbiome [1].","Choose a pet for the companionship, not to prevent allergies; pooled birth-cohort data found no clear effect either way [5]."],"cautions":["Raw milk is unsafe for 'building immunity': it figured in the farm-asthma link [3], yet unpasteurized dairy caused about 840 times more outbreak illnesses in one US model [6].","Avoid raw milk in pregnancy and for young children; see [the raw milk debate](/raw-milk/the-raw-milk-debate).","For persistent wheeze, eczema or possible food allergy, see a pediatrician rather than relying on dirt, pets or farm visits, none an established way to prevent asthma or allergy [5]."],"simpleText":"The \"hygiene hypothesis\" says children raised too clean miss germs that train the immune system. A 2016 expert review calls the name misleading: there is no good evidence that everyday cleaning is behind the rise in allergy [1]. What seems to matter is early contact with a wide variety of microbes from farms, animals, soil and other people. Children raised on traditional farms have less asthma and hay fever [2][3]. Handwashing still prevents real infections [4].","mediumText":"How it works\n\nWhere the idea came from\n\nIn 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 allergy [7]. The pattern has held up in far larger later studies, mainly in wealthier countries, although severe asthma and eczema were more common in larger families [8]. The idea was soon shortened to the \"hygiene hypothesis.\"\n\nWhy researchers want a new name\n\nClean 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 allergy [1]. More likely suspects are urban living, less time outdoors, changes in diet and antibiotic use, all of which change the microbes we meet and carry [1].\n\nThe 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 inflammation [9].\n\nWhat farms and pets show\n\nChildren who grow up on traditional farms, with cows, straw and farm milk, have less asthma, hay fever and eczema than other rural children [3]. Their bedroom dust held a wider variety of microbes, and that variety explains a good share of the difference [2]. In the United States, Amish children had far less asthma than Hutterite children despite similar genes and lifestyles, and the two groups farm differently [10].\n\nPets 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 asthma [5].\n\nWhat still protects us\n\nWashing hands with soap cut diarrhea risk by an estimated 42-47% in a pooled analysis of 17 studies [4]. The expert panel's answer is \"targeted hygiene\": clean where it stops infection, such as hands, food, toilets and wounds, and relax about everyday dirt [1]. For the microbe side of the story, see Dirt, Soil Microbes, and Immune Health.\n\nWhat to take from this\n\nThese 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.","deepText":"The evidence in detail\n\nFarm exposure: two large cross-sectional studies\n\nThe 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 7,682 [3]. 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 children [3]. Contact with cows, straw and farm milk accounted for the asthma effect, but no measured exposure fully explained the effect on hay fever and sensitization [3]. Limitations: parents reported the exposures, and farming families may differ from others in ways no one measured.\n\nA companion analysis asked which microbes might be responsible [2]. 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 effect [2]. Because exposure and disease were measured at the same time, cause and effect cannot be separated.\n\nSiblings and family size\n\nThe 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 countries [8]. 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 countries [8]. By contrast, symptoms of severe asthma and severe eczema rose with family size in both age groups [8]. The study is questionnaire-based and cross-sectional, so it shows patterns, not mechanisms.\n\nSame genes, different farming\n\nStein and colleagues compared 60 Amish and Hutterite children in the US [10]. 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 homes [10]. 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 Trif [10]. Limitations: only 60 children from two communities, and mouse findings do not prove the same mechanism in people.\n\nPets and early animal contact\n\nIn 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 link [12]. Both studies are observational. Families with allergic children may avoid pets, which could produce part of the pattern without any protection.\n\nConflicting results\n\nThe broadest pet analysis pooled individual data from 11 European birth cohorts that recruited more than 22,000 children in the 1990s [5]. 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 rhinitis [5]. 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.\n\nHygiene and infection\n\nCurtis and Cairncross pooled 7 intervention studies, 6 case-control, 2 cross-sectional and 2 cohort studies [4]. 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 result [4]. On raw milk, a US outbreak model estimated that unpasteurized dairy products cause 840 times more illnesses and 45 times more hospitalizations than pasteurized ones [6].\n\nWhat the evidence cannot show\n\nNone 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.\n\nBottom line (confidence 3 of 5)\n\nFarm 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.","deepTruncated":false,"references":[{"id":"1","title":"Time to abandon the hygiene hypothesis: new perspectives on allergic disease, the human microbiome, infectious disease prevention and the role of targeted hygiene","authors":"Bloomfield SF, Rook GA, Scott EA, et al.","year":2016,"journal":"Perspect Public Health","pmid":"27354505","url":"https://pubmed.ncbi.nlm.nih.gov/27354505/"},{"id":"2","title":"Exposure to environmental microorganisms and childhood asthma","authors":"Ege MJ, Mayer M, Normand AC, et al.","year":2011,"journal":"N Engl J Med","pmid":"21345099","url":"https://pubmed.ncbi.nlm.nih.gov/21345099/"},{"id":"3","title":"Protection from childhood asthma and allergy in Alpine farm environments-the GABRIEL Advanced Studies","authors":"Illi S, Depner M, Genuneit J, et al.","year":2012,"journal":"J Allergy Clin Immunol","pmid":"22534534","url":"https://pubmed.ncbi.nlm.nih.gov/22534534/"},{"id":"4","title":"Effect of washing hands with soap on diarrhoea risk in the community: a systematic review","authors":"Curtis V, Cairncross S","year":2003,"journal":"Lancet Infect Dis","pmid":"12726975","url":"https://pubmed.ncbi.nlm.nih.gov/12726975/"},{"id":"5","title":"Does pet ownership in infancy lead to asthma or allergy at school age? Pooled analysis of individual participant data from 11 European birth cohorts","authors":"Lødrup Carlsen KC, Roll S, Carlsen KH, et al.","year":2012,"journal":"PLoS One","pmid":"22952649","url":"https://pubmed.ncbi.nlm.nih.gov/22952649/"},{"id":"6","title":"Outbreak-Related Disease Burden Associated with Consumption of Unpasteurized Cow's Milk and Cheese, United States, 2009-2014","authors":"Costard S, Espejo L, Groenendaal H, et al.","year":2017,"journal":"Emerg Infect Dis","pmid":"28518026","url":"https://pubmed.ncbi.nlm.nih.gov/28518026/"},{"id":"7","title":"Hay fever, hygiene, and household size","authors":"Strachan DP","year":1989,"journal":"BMJ","pmid":"2513902","url":"https://pubmed.ncbi.nlm.nih.gov/2513902/"},{"id":"8","title":"Siblings, asthma, rhinoconjunctivitis and eczema: a worldwide perspective from the International Study of Asthma and Allergies in Childhood","authors":"Strachan DP, Aït-Khaled N, Foliaki S, et al.","year":2015,"journal":"Clin Exp Allergy","pmid":"24912652","url":"https://pubmed.ncbi.nlm.nih.gov/24912652/"},{"id":"9","title":"Regulation of the immune system by biodiversity from the natural environment: an ecosystem service essential to health","authors":"Rook GA","year":2013,"journal":"Proc Natl Acad Sci U S A","pmid":"24154724","url":"https://pubmed.ncbi.nlm.nih.gov/24154724/"},{"id":"10","title":"Innate Immunity and Asthma Risk in Amish and Hutterite Farm Children","authors":"Stein MM, Hrusch CL, Gozdz J, et al.","year":2016,"journal":"N Engl J Med","pmid":"27518660","url":"https://pubmed.ncbi.nlm.nih.gov/27518660/"},{"id":"11","title":"Early Exposure to Dogs and Farm Animals and the Risk of Childhood Asthma","authors":"Fall T, Lundholm C, Örtqvist AK, et al.","year":2015,"journal":"JAMA Pediatr","pmid":"26523822","url":"https://pubmed.ncbi.nlm.nih.gov/26523822/"},{"id":"12","title":"Early exposure to cats, dogs and farm animals and the risk of childhood asthma and allergy","authors":"Ojwang V, Nwaru BI, Takkinen HM, et al.","year":2020,"journal":"Pediatr Allergy Immunol","pmid":"31829464","url":"https://pubmed.ncbi.nlm.nih.gov/31829464/"}],"wordCount":1383,"source":{"path":"src/content/topics/hygiene-hypothesis/too-clean-the-hygiene-hypothesis.md"},"citeAs":"“Is being too clean making kids sick (dirt, farms, siblings, pets)?.” What's Healthy and Why. https://whatshealthyandwhy.com/hygiene-hypothesis/too-clean-the-hygiene-hypothesis","notice":"General information backed by peer-reviewed research, not medical advice."}