Why Are Allergies Increasing Worldwide?

The Hygiene Hypothesis: Too Clean for Our Own Good?

The hygiene hypothesis, first proposed by David Strachan in 1989, suggests that reduced exposure to infectious agents and microorganisms in early childhood — a result of improved sanitation, smaller family sizes, antibiotic use, and indoor living — prevents normal immune maturation. Without adequate microbial challenge, the immune system defaults to Th2-dominant responses that drive allergic disease.

The 'old friends' hypothesis, a refinement of the hygiene hypothesis, proposes more specifically that reduced exposure to parasites, commensal bacteria, and harmless environmental microorganisms that co-evolved with humans disrupts immune regulation. The regulatory T cell networks that normally prevent allergic overreaction fail to develop properly without appropriate early microbial stimulation.

Evidence supporting this includes higher allergy rates in urban versus rural children, lower rates in children raised on farms with livestock exposure, the protective effect of older siblings, and associations between early antibiotic use and increased allergy and asthma risk in multiple prospective studies.

The Gut Microbiome's Role in Allergy Development

The gut microbiome — the community of trillions of microorganisms living in the digestive tract — plays a central role in immune education during the first months and years of life. Reduced microbial diversity in the infant gut, associated with C-section delivery, formula feeding, antibiotic use, and low dietary fiber, is consistently linked with higher risks of developing food allergy, atopic dermatitis, and asthma.

Studies of rural communities in Finland and Karelia (Russia) with dramatically different allergy rates despite similar genetics have demonstrated that gut microbiome composition differs significantly. Infants in lower-allergy environments have greater abundance of Bifidobacterium and Lactobacillus species and higher levels of short-chain fatty acids that support regulatory immune development.

Climate Change, Pollution, and Longer Pollen Seasons

Climate change contributes to rising allergy burden through multiple mechanisms. Pollen seasons have lengthened by approximately 20 days since 1990, and total pollen production has increased by 21%. Higher atmospheric CO2 accelerates plant growth and pollen output. Warmer winters allow allergenic plants to establish in new geographic regions.

Air pollution — particularly diesel exhaust particles — increases the allergenic potency of pollen by binding to allergen proteins and enhancing their ability to penetrate mucous membranes. Ozone, particulate matter, and nitrogen dioxide increase airway inflammation and lower the threshold for allergic sensitization. Urban populations show higher sensitization rates than rural populations exposed to similar pollen levels.

Diet, Early Allergen Avoidance, and Food Allergy

Dietary changes over the past 50 years — reduced fiber intake, increased processed food consumption, lower omega-3 fatty acid intake — may impair the gut microbiome and immune regulation. High-fat, low-fiber Western diets reduce the short-chain fatty acid production that supports regulatory T cell development.

Paradoxically, official advice to avoid peanuts and other allergenic foods in early infancy — which prevailed from about 2000 to 2015 — may have contributed to rising peanut allergy rates. The LEAP trial demonstrated that early introduction of peanut to high-risk infants reduces peanut allergy by 80%, and guidelines were reversed in 2017. Early introduction of diverse allergenic foods is now recommended for most infants.