“Cold weather weakens your immune system and makes you more likely to get sick.”

• A 2022 study in the Journal of Allergy and Clinical Immunology (Huang, Bleier and colleagues, Massachusetts Eye and Ear/Northeastern) found that lowering nasal tissue temperature by roughly 5°C — the change produced by breathing cold air — cut the release of antiviral extracellular vesicles from nasal epithelium by about 40% and reduced their ability to bind and neutralize viruses. This is direct mechanistic evidence that cold air blunts the nose's first-line innate immune defense.
• Foxman et al., PNAS 2015, showed that rhinovirus replicates better at 33°C (cooler airway temperature) than at 37°C, largely because the interferon-mediated innate antiviral response of airway cells is weaker at cooler temperatures. Cooling the nasal passages therefore plausibly gives common-cold viruses an advantage.
• Cold air causes vasoconstriction in the nasal mucosa and slows mucociliary clearance, reducing the rate at which inhaled pathogens are physically removed from the airway.
• Cold weather is accompanied by low absolute humidity, and Kudo et al. (PNAS 2019) showed low ambient humidity impairs airway barrier function, mucociliary clearance and interferon-stimulated antiviral responses in mice, increasing susceptibility to influenza.
• Winter reduces sunlight exposure and vitamin D synthesis; meta-analyses of randomized trials (e.g., Martineau et al., BMJ 2017) indicate vitamin D supplementation modestly reduces acute respiratory infections, especially in deficient individuals — a plausible seasonal immune pathway.
• Epidemiologically, influenza, RSV and most common-cold coronaviruses show strong winter peaks in temperate zones, consistent with some cold-related increase in susceptibility and/or transmission.
• A frequently cited 2005 study by Johnson and Eccles (Family Practice) reported that volunteers whose feet were chilled in cold water developed more self-reported cold symptoms over the following days than controls, suggesting chilling may trigger symptomatic infection in people already carrying a virus.

• Cold weather alone cannot make you sick: colds and flu are caused by viruses, and without exposure to a pathogen, being cold produces no infection. The popular version of this claim ('going out with wet hair gives you a cold') is a myth.
• Classic experimental studies contradict the folk version. Douglas and colleagues (New England Journal of Medicine, 1968) inoculated volunteers with rhinovirus and exposed them to cold environments (including near-freezing temperatures and wet conditions) and found no increase in infection rate or illness severity. Similar chilling experiments at the UK Common Cold Unit in Salisbury also failed to show increased susceptibility.
• Most public-health and academic sources attribute winter seasonality primarily to behavior and environment rather than immune weakening: people crowd indoors with poor ventilation, school terms concentrate children, and low humidity plus low temperature increase the survival and aerosol persistence of influenza and other viruses outside the body.
• The demonstrated cold-related immune effect is local (nasal epithelium) and temporary, not a systemic 'weakened immune system.' There is little evidence that ordinary cold exposure suppresses adaptive immunity or leukocyte function in healthy people; severe hypothermia is a different matter.
• The Johnson & Eccles foot-chilling study relied on self-reported symptoms, had no virological confirmation, and has not been robustly replicated, so it is weak evidence.
• Not all respiratory or infectious seasonality follows cold: enteroviruses, some parainfluenza serotypes and many other infections peak in summer, and respiratory viruses also circulate seasonally in warm tropical regions (often tied to rainy seasons), undermining a simple 'cold = sick' model.
• Some evidence points the other way for brief cold exposure: short cold-water immersion transiently increases circulating leukocytes and catecholamines, and a randomized trial of cold showers (Buijze et al., PLOS ONE 2016) found fewer sickness absence days, though not fewer reported illnesses.

78/100 — the lab’s own confidence in the score above, not a second reading of the claim.

Proceedings of the National Academy of Sciences (Foxman et al., Yale)journal · 93 · Journal of Allergy and Clinical Immunology (Huang/Bleier et al., 2022, nasal extracellular vesicles and cold exposure)journal · 88 · Proceedings of the National Academy of Sciences (Kudo et al., low humidity and influenza susceptibility)journal · 92 · The BMJ (Martineau et al., vitamin D and acute respiratory infections meta-analysis)journal · 90 · New England Journal of Medicine (Douglas et al., 1968, cold exposure and rhinovirus colds — no effect demonstrated)journal · 92 · U.S. Centers for Disease Control and Prevention (influenza seasonality)gov · 90 · Harvard Health Publishing, Harvard Medical Schooledu · 85 · Johns Hopkins Medicineedu · 87 · Mayo Clinicedu · 87 · NIH News in Health (National Institutes of Health)gov · 88 · PLOS ONE (Buijze et al., cold shower randomized trial)journal · 78

• Cold exposure can impair local nasal antiviral defenses: a Journal of Allergy and Clinical Immunology study found that cooling the nasal cavity reduced extracellular-vesicle-mediated antiviral activity, a plausible mechanism for increased susceptibility to respiratory viruses.
• Cooler temperatures in the upper airway can favor rhinovirus infection: a PNAS study found rhinovirus replicated better at cooler nasal-cavity-like temperatures than at core body temperature, partly because innate antiviral responses were less effective.
• Cold, dry conditions can increase respiratory-virus transmission independent of human behavior: a PLoS Pathogens guinea-pig influenza study found transmission was more efficient at low temperature and low humidity.
• Government health sources note that common colds are more common in winter and spring, consistent with cold-season conditions increasing infection risk, though not necessarily by broadly weakening the whole immune system.

• The claim is overstated if it means cold air alone causes illness: CDC, MedlinePlus, and NIAID identify viruses as the cause of common colds; exposure to pathogens is required.
• The strongest evidence points to local nasal/airway effects and virus-transmission advantages in cold/dry air, not a generalized weakening of the entire immune system.
• Respiratory infections occur year-round, so cold weather is neither necessary nor sufficient for getting sick.
• Winter illness patterns also reflect non-immune factors such as indoor crowding, closer contact, and environmental effects on virus stability and aerosol spread.

86/100 — the lab’s own confidence in the score above, not a second reading of the claim.

Journal of Allergy and Clinical Immunologyjournal · 91 · Proceedings of the National Academy of Sciencesjournal · 90 · PLoS Pathogensjournal · 89 · Centers for Disease Control and Preventiongov · 93 · MedlinePlus / National Library of Medicinegov · 92 · National Institute of Allergy and Infectious Diseasesgov · 92

• A 2022 study published in The Journal of Allergy and Clinical Immunology found that dropping the temperature inside the nose by 5 degrees Celsius reduces the secretion of virus-fighting extracellular vesicles by nearly 50%, thereby weakening the local immune response in the nasal cavity.
• Research from Yale University demonstrated that the rhinovirus (the most frequent cause of the common cold) replicates more efficiently at the cooler temperatures found in the nasal cavity during cold weather (around 33°C) compared to core body temperature (37°C).
• Cooler temperatures also slow down the immune system's antiviral responses, such as the production of interferons, allowing viruses to multiply faster.
• Cold, dry winter air prolongs the lifespan of expelled viral particles in aerosols and droplets, allowing them to remain airborne longer and transmit more easily.
• Cold weather drives people indoors where ventilation is poorer and physical proximity is closer, which epidemiologically increases the likelihood of viral transmission.

• Cold weather itself does not spontaneously generate illness; exposure to a pathogen (like a virus or bacteria) is strictly required to get sick.
• Systemic immune function (the overall immune system throughout the rest of the body) is not significantly compromised by standard ambient cold exposure; the weakening effect is heavily localized to the upper respiratory mucosa.
• Historically, public health messaging pushed back against this claim as an 'old wives' tale' (e.g., getting a chill or going outside with wet hair directly causing a cold), emphasizing that viruses, not temperatures, are the root cause of infections.

95/100 — the lab’s own confidence in the score above, not a second reading of the claim.

The Journal of Allergy and Clinical Immunologyjournal · 95 · National Institutes of Healthgov · 98 · PBS NewsHournews · 90

• Cold air can impair local immune defenses in the nose and upper airways. Experimental and clinical work summarized by major health outlets reports that even a modest drop (about 5°C) in nasal tissue temperature can reduce innate immune responses in the nasal mucosa by roughly half, allowing more respiratory viruses to enter cells and replicate.
• Cold, dry air can damage or dry out mucus membranes and impair mucociliary clearance in the respiratory tract, which are key first-line defenses against pathogens. When the mucus and cilia in the nose and throat are less efficient, viruses that are inhaled are more likely to reach and infect target cells.
• Cold exposure can divert physiological resources toward maintaining core temperature, which some clinicians and researchers describe as leaving fewer resources for immune responses. This effect is often cited by hospital and clinic physicians as one reason infections may be harder to fight off when it is chilly and damp.
• Several epidemiological and mechanistic studies link lower ambient temperatures with increased incidence of respiratory infections (e.g., colds, influenza, pneumonia). These studies note that susceptibility to respiratory viruses increases when temperature and humidity decrease, partly due to impaired mucociliary function and increased bronchial reactivity.
• Large health organizations and academic medical centers explain winter surges in colds and flu partly by noting that cooler temperatures can weaken local immune responses in the airways and reduce blood flow to the lining of the nose, throat, and lungs, which may dampen immune responses and make individuals more susceptible to infection.
• Some research and expert summaries indicate that immune cells can be less active or ‘more sluggish’ in cold temperatures, and that cold-induced vasoconstriction may reduce the delivery of white blood cells to the respiratory tract, further compromising defenses against respiratory infections.

• Respiratory infections such as the common cold, influenza, and COVID-19 are caused by viruses and other pathogens, not by cold weather itself. Major medical sources emphasize that you cannot ‘catch a cold’ simply by being cold; exposure to infectious agents is required.
• Several reputable health organizations state explicitly that cold weather alone does not directly cause illness. They describe cold as an indirect factor that may modestly increase risk or foster conditions that favor transmission, but not as a direct cause of sickness in the absence of pathogens.
• Much of the increased winter illness burden is explained by behavioral and environmental factors rather than cold-induced global immune suppression: people spend more time indoors in close contact, ventilation is poorer, and viruses survive and transmit more efficiently in cool, dry indoor air.
• Public health and academic sources often frame the effect as *localized* or *situational* immune changes (e.g., in the nasal passages) rather than a generalized weakening of the entire immune system. They caution against the overstatement that cold weather broadly ‘weakens your immune system’ in a way that inevitably makes you sick.
• Fact-check style explanations and evidence-based health articles usually conclude that the traditional belief that cold weather by itself makes people sick is overstated or partly wrong: the cold may slightly increase susceptibility and viral spread, but contact with infectious agents and other factors are the primary determinants.
• Some experts note that healthy people with adequate clothing and good overall health can tolerate cold exposure without significant immune compromise, and that the effect of ordinary outdoor cold exposure on systemic immunity is likely modest compared with factors like vaccination status, sleep, nutrition, and chronic disease.

90/100 — the lab’s own confidence in the score above, not a second reading of the claim.

Cleveland Clinicedu · 92 · Cleveland Clinicedu · 92 · Healthlinenews · 82 · Healthlinenews · 82 · PBS NewsHournews · 90 · Bupaother · 80 · National Library of Medicine (PMC)journal · 95 · National Library of Medicine (PMC)journal · 95 · Mayo Clinicedu · 93 · Northwestern Medicineedu · 90 · GoHealth Urgent Careother · 78 · USA Todaynews · 80

• A 2015 PNAS study found rhinovirus (a leading cause of the common cold) replicates more efficiently and innate antiviral defenses such as interferon signaling are less effective at cooler temperatures typical of the nasal cavity in cold weather.
• Colder and drier air can impair mucociliary clearance and reduce local innate immune barriers in the upper respiratory tract, making viral establishment easier.
• Respiratory virus incidence (colds, influenza) reliably peaks in winter in temperate climates, coinciding with cold weather.
• Some experimental work indicates modest effects of cold stress on certain immune functions and on nasal extracellular-vesicle antiviral responses.

• Colds and flu are caused by viruses, not by low temperature; pathogen exposure is required.
• Winter peaks are driven heavily by indoor crowding, reduced ventilation, school calendars, and holiday mixing rather than temperature weakening immunity as a whole.
• Major clinical sources (e.g., Mayo Clinic) state there is no evidence that simply being chilled or going outside in cold weather causes a cold.
• Ordinary cold-weather exposure does not meaningfully weaken the systemic immune system in healthy people; any temperature effects appear localized and modest.
• The same viruses circulate year-round; seasonality has multiple non-immune explanations.

78/100 — the lab’s own confidence in the score above, not a second reading of the claim.

PubMed/National Library of Medicinegov · 95 · Proceedings of the National Academy of Sciencesjournal · 96 · National Institutes of Healthgov · 95 · Centers for Disease Control and Preventiongov · 98 · Mayo Clinicother · 92