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

• Laboratory work by Foxman and colleagues (Yale, PNAS 2015) showed that rhinovirus replicates more efficiently at cooler airway temperatures (~33°C, typical of the nose in cold air) than at core body temperature (37°C), largely because the interferon-based innate immune response of airway cells is blunted at the lower temperature — i.e., cooling genuinely weakens a local antiviral defense.
• A 2022/2023 study led by researchers at Mass Eye and Ear/Harvard and Northeastern, published in the Journal of Allergy and Clinical Immunology, found that dropping nasal tissue temperature by about 5°C (equivalent to stepping into cold air) cut the release of antiviral extracellular vesicles from nasal cells by roughly 40% and reduced their virus-neutralizing capacity — a direct mechanism by which cold exposure impairs immunity at the site where most respiratory viruses enter.
• Cold, dry air slows mucociliary clearance and dries nasal mucosa, reducing the physical barrier that traps and removes inhaled pathogens.
• Cold-induced vasoconstriction in the upper airway reduces delivery of white blood cells and immune mediators to the nasal mucosa (Eccles, Rhinology, 2002), providing a plausible physiological basis for the claim.
• A randomized study by Johnson and Eccles (Family Practice, 2005) found that volunteers whose feet were chilled in cold water reported significantly more cold symptoms in the following days than controls.
• Epidemiologically, influenza, RSV and common colds show strong cold-season peaks in temperate climates (CDC, WHO surveillance data), consistent with cold/dry conditions favoring infection and transmission.
• Low absolute humidity, which accompanies cold weather, increases influenza virus survival in aerosols and transmission in animal models (Lowen et al., PLoS Pathogens, 2007), and low humidity has been shown to impair host antiviral responses in mice (Kudo et al., PNAS, 2019).

• Cold weather by itself cannot make you sick — a pathogen is required. The statement's shorthand can reinforce the folk belief that chilling alone causes illness.
• Classic human-challenge experiments (e.g., Douglas et al., NEJM, 1968; later work at the UK Common Cold Unit) found that chilling volunteers did not measurably increase their rate of infection or illness severity after rhinovirus inoculation, so the effect is not robust in every experimental design.
• 'Weakens your immune system' is imprecise: the best-supported effects are localized (nasal/airway innate immunity, reduced mucosal clearance) rather than a demonstrated systemic immunosuppression. Some data suggest brief cold exposure can transiently increase circulating leukocytes and certain immune markers.
• Winter respiratory-illness peaks are multifactorial: indoor crowding, poor ventilation, school terms, and reduced vitamin D/UV all contribute, so cold temperature is not the sole driver of increased illness.
• Older public-health messaging and some fact-checks have framed 'cold weather makes you sick' as a myth, reflecting the fact that the mechanistic evidence only firmed up in the last decade.

80/100 — the lab’s own confidence in the score above, not a second reading of the claim.

Yale University (reporting Foxman et al., PNAS 2015)edu · 92 · Journal of Allergy and Clinical Immunology (Huang et al., nasal extracellular vesicles and cold exposure)journal · 90 · Mass Eye and Ear / Harvard Medical School (press release on cold-temperature nasal immunity study)edu · 85 · Family Practice (Johnson & Eccles, acute cooling of the feet and common cold symptoms, 2005)journal · 78 · New England Journal of Medicine (Douglas et al., cold exposure and rhinovirus colds, 1968)journal · 88 · PLoS Pathogens (Lowen et al., influenza transmission depends on humidity and temperature, 2007)journal · 90 · PNAS (Kudo et al., low ambient humidity impairs antiviral defense, 2019)journal · 92 · U.S. Centers for Disease Control and Prevention (flu seasonality)gov · 93 · Harvard Health Publishing (why respiratory infections rise in winter)edu · 85 · UK National Health Service (common cold causes)gov · 88

• Cold-season respiratory infections are not just folklore: CDC notes that common colds are more common in winter and spring, and that they are caused by respiratory viruses that spread between people, so cold seasons coincide with higher exposure and transmission opportunities.
• Laboratory evidence supports a biological mechanism: Foxman et al. in PNAS found rhinovirus replicated better at cooler nasal-airway temperatures partly because antiviral innate immune responses were less effective at 33°C than at core body temperature.
• A 2023 Journal of Allergy and Clinical Immunology study reported that cold exposure lowered nasal temperature and impaired extracellular-vesicle-mediated antiviral defenses in the nose, supporting the idea that cold air can weaken local front-line respiratory immune defenses.
• Cold-weather conditions can also favor viruses indirectly: PNAS research found low humidity impaired airway barrier function and innate resistance to influenza in mice, while PLOS Pathogens research found influenza transmission in guinea pigs was more efficient at low temperature and low humidity—conditions common in winter.

• The wording is too broad if read as 'cold air alone gives you a cold' or 'cold weather generally suppresses the whole immune system.' CDC is clear that common colds are caused by viruses, not temperature itself.
• Much of the increased winter illness risk comes from multiple factors besides immune weakening, including more indoor crowding, closer contact, school/work patterns, and environmental effects on virus survival and transmission.
• The strongest human evidence concerns local nasal/airway innate defenses, not a blanket weakening of the entire immune system. Normal brief cold exposure in healthy people is not the same as severe cold stress or hypothermia.
• Because infection still requires exposure to a pathogen, cold weather by itself does not guarantee sickness; it increases risk under common real-world conditions.

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

Centers for Disease Control and Preventiongov · 95 · Proceedings of the National Academy of Sciencesjournal · 92 · Journal of Allergy and Clinical Immunologyjournal · 90 · National Institutes of Healthgov · 92 · Proceedings of the National Academy of Sciencesjournal · 90 · PLOS Pathogensjournal · 88

• A breakthrough 2022 study published in The Journal of Allergy and Clinical Immunology demonstrated that inhaling cold air lowers the temperature inside the nasal cavity, which physically impairs the local immune response. Specifically, the cold reduces the secretion of extracellular vesicles (EVs) that typically swarm and neutralize respiratory viruses by up to 50%.
• Many respiratory pathogens, such as rhinoviruses (the most common cause of the common cold), have been shown to replicate much more efficiently at the cooler temperatures found in the nasal cavity during cold weather.
• Cold weather indirectly increases the likelihood of illness by driving people indoors into poorly ventilated spaces where viruses spread more easily, and by decreasing absolute humidity, which allows aerosolized virus particles to remain stable and infectious in the air for longer periods.

• Cold weather alone cannot cause an infection; exposure to a pathogen (such as a virus or bacteria) is strictly required to get sick.
• The statement's phrasing implies a systemic weakening of the entire immune system (e.g., T-cells and B-cells throughout the body). In reality, the direct immunosuppressive effect of cold air is localized specifically to the upper respiratory tract (the nose).
• Historically, health agencies and doctors have labeled the old adage that 'being cold gives you a cold' as a myth, emphasizing that behavioral factors (indoor crowding) and viral prevalence are the main drivers of winter sickness, rather than systemic immune failure.

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 · CNNnews · 85 · Mayo Clinicother · 95

• Several medical sources state that cold weather can impair aspects of immune defense, especially in the respiratory tract, making infections more likely when viruses are present. For example, Cleveland Clinic notes that cold, wet conditions can weaken the immune system by forcing the body to work harder to stay warm and by damaging mucus membranes that normally trap pathogens, allowing viruses easier entry into the body.[1][3][9][6]
• Research summarized by Healthline reports that cold temperatures in the nose reduce local immune responses: a drop of about 5°C in nasal tissue temperature was associated with about a 50% reduction in certain antiviral immune responses, which can increase susceptibility to respiratory viruses.[2][3][8][12]
• A study discussed by AccuWeather and PBS, based on research published in The Journal of Allergy and Clinical Immunology, found that lowering the temperature inside the nose by about 5°C killed nearly half of the antiviral and antibacterial particles (extracellular vesicles) in the nostrils, thereby substantially reducing the nose’s ability to fight respiratory infections; experts quoted in these reports explicitly state that this reduced local immunity is one reason cold air is associated with more viral infections.[8][12]
• Harvard Health explains that cold weather does not directly cause illness but may foster it because cold air reduces blood flow to the lining of the nose, throat, and lungs, which can dampen immune responses; they also note that some cold and flu viruses multiply and spread more easily in lower temperatures and humidity, compounding the effect.[10]
• National Geographic, summarizing work from Yale researchers, describes how cold, dry air in winter can tax host immune responses: inhaling dry, cold air can impair the mucosal defenses of the respiratory tract and, combined with other winter factors (e.g., lack of sunlight), reduces the body’s ability to fight respiratory viruses like influenza and SARS‑CoV‑2.[13][14]
• Age UK, a health-focused charity working with medical sources, states that people are more likely to become ill during winter partly because some viruses survive more easily in cold conditions and because the cold can make it more difficult for the immune system to fight off illnesses, suggesting a weakening of immune effectiveness under cold exposure.[11]
• Several health organizations note that cold temperatures and dry air can damage or dry out mucus membranes and cilia in the nose and throat, reducing their efficiency as a first line of defense, which increases vulnerability if one is exposed to viruses; this mechanistic explanation supports the idea that cold weather can weaken parts of the immune barrier and thus increase the likelihood of getting sick when pathogens are present.[1][6][9][7]
• Some clinical and public health sources mention that cold temperatures cause vasoconstriction (narrowing of blood vessels), which can reduce the delivery of immune cells (such as white blood cells) to the respiratory tract, potentially decreasing local immune surveillance and making respiratory infections somewhat more likely in cold environments.[5][10]

• A peer‑reviewed review article in Medicine & Science in Sports & Exercise examining human immune responses to cold exposure concluded that, based on available data, there is no robust support for the concept that cold exposure per se depresses overall immune function or clearly increases upper respiratory tract infection incidence; moderate acute cold exposure appeared to have no detrimental effect on the innate component of the immune system.[4]
• An NIH‑hosted review on temperature‑dependent viral tropism notes classic experimental studies (e.g., by Andrewes and Dowling) in which deliberate chilling of individuals did not meaningfully weaken systemic immune defenses, suggesting that simply being cold does not broadly suppress immune function in humans.[15]
• Healthline and other medical sources emphasize that cold weather itself does not directly cause illness; viruses and bacteria do. These sources argue that while cold air can weaken local nasal immune responses, you still must be exposed to pathogens to get sick, and the main drivers of winter respiratory illness are increased viral activity, indoor crowding, and transmission dynamics, rather than a generalized weakening of the entire immune system by cold temperatures.[3][6][10]
• Several health organizations explicitly state that cold weather alone does not make you ill; it may increase your chances of becoming ill primarily by changing behavior (more indoor close contact) and by providing environmental conditions that help viruses survive and spread, rather than by substantially and globally weakening the immune system.[6][9][10]
• The sports‑medicine review and temperature‑tropism review also highlight that many claims about cold exposure and immune suppression come from limited or older data and that available human studies do not unequivocally support the notion that cold exposure in otherwise healthy individuals significantly suppresses systemic immune function or dramatically increases infection risk absent other factors like pathogen exposure and crowding.[4][15]

88/100 — the lab’s own confidence in the score above, not a second reading of the claim.

Cleveland Clinicedu · 92 · Healthlinenews · 80 · Healthlinenews · 80 · Medicine & Science in Sports & Exercise (LWW)journal · 94 · GoHealth Urgent Careother · 75 · Bupaother · 82 · Nutrisenseother · 70 · AccuWeathernews · 75 · Cleveland Clinicedu · 92 · Harvard Health Publishingedu · 93 · Age UKother · 80 · PBS NewsHournews · 90

• Cooler nasal temperatures (typical in cold weather) allow rhinoviruses to replicate more efficiently and weaken local interferon and innate antiviral responses (Foxman et al., PNAS 2015).
• A 2022 study found breathing cold air (~4°C) sharply reduced nasal extracellular vesicles that trap and kill viruses, impairing upper-airway defenses.
• Respiratory viruses remain infectious longer in cold, dry air, and winter crowding indoors increases transmission.
• Seasonal vitamin D decline and drier mucous membranes are additional plausible cofactors that raise infection risk.

• Infectious illnesses require a pathogen; cold exposure alone does not create viruses or bacteria.
• Classic challenge studies (Common Cold Unit and similar) generally found that chilling people did not raise infection rates when virus exposure was controlled.
• Effects are mostly local to the nose, not systemic weakening of the whole immune system in healthy people.
• Major medical sites have long labeled “you catch a cold just from being cold” a myth, stressing viruses, hygiene, and vaccination as the real drivers.
• The same viruses circulate in warm seasons and climates; temperature is a contributing factor, not the cause.

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

CDCgov · 96 · PNASjournal · 92 · Mayo Clinicother · 88