Cold Weather and Catching Colds

Breathing · §9

Three colds a winter, and the worst lands the week you couldn't take time off. Your grandmother said it was wet hair and the draft on your neck. Your doctor said weather has nothing to do with it. Both were wrong — but in interesting, fixable ways. Cold air really does drop the nose's antiviral firepower; dry heated rooms really do let exhaled virus linger for half an hour instead of seconds. The virus still has to find you. But the season tilts the odds, and most of the tilt is in your house, not outside it.

Know · As-needed Evidence Emerging Chapter Breathing

You can't catch a cold from cold weather alone — the virus has to be there. But the dry, heated air winter brings makes the viruses you do meet stick more easily, and a few cheap moves in your own house claw most of that back. A humidifier kept around 40 to 60 percent, a daily vitamin D pill, the hand-washing you'd already do when something's going around. Fewer sick weeks, milder when they hit, less of the wrong bug landing on someone who can't shake it.

A cold is a viral infection. The biggest single cause is rhinovirus — about a third to a half of adult colds — with the rest split between seasonal coronaviruses, RSV, parainfluenza, and whatever happens to be circulating Moriyama et al. 2020. The necessary cause is exposure: a virion has to get into your airway. Cold weather alone, in a sterile bubble, does nothing.

What cold weather does do is change the odds that any given viral exposure turns into a symptomatic cold. Two mechanisms do most of the work.

The first is in your nose. The front of your nasal cavity sits at about 32 to 33°C — several degrees cooler than your core — and rhinovirus evolved to prefer it that way. The closer the nose drops toward the virus's preferred temperature, the slower the local immune machinery runs: the alarm chemicals your cells release when they spot a virus (called type I interferons) lose potency, and infected cells take longer to self-destruct and clear out Foxman et al. 2015. Stand outside in genuinely cold air and the front of the nose drops another five degrees or so — and a separate antiviral mechanism, a swarm of tiny decoy bubbles your nasal cells fire at incoming virions, drops with it.

The second mechanism is in the air around you. Heated winter rooms with no humidification commonly sit at 15 to 20 percent relative humidity; outdoor air during a January cold snap holds essentially no moisture at all. At those levels, the mucus blanket lining your airway dries out, the tiny hairs that sweep virions back up toward the throat slow down, and the small respiratory droplets a sick coworker exhales evaporate in seconds into virion-laden particles that hang in still indoor air for half an hour or longer Yang & Marr 2011, Tellier 2009. The same particles in a humid room fall out of the air fast.

What the studies actually show

The mechanism literature is multi-paper and converging. The human-outcome literature is thinner but consistent.

The biggest population-scale evidence is the seasonality itself. In countries with real winters, respiratory infections cluster in autumn and winter every year, in the same shape, across decades and across viruses that have little in common except your airway Moriyama et al. 2020. Behaviour drives a lot of it — people pack indoors, kids cycle viruses through schools, families travel for holidays. Environment drives the rest — cold air on the nose, dry indoor heat. The relative weights are still debated; neither is zero.

On the humidity side, the cleanest experiment is in guinea pigs. Lowen and colleagues infected animals and tracked transmission to cage-mates across a temperature and humidity grid. At 5°C and 20 to 35 percent humidity, transmission was fast and frequent; at 30°C it collapsed to zero, at the same humidity Lowen et al. 2007. The pattern has held in replications and is the basis for the case that humidity is doing real work in winter seasonality.

For the supplement that actually has an outcome trial: vitamin D.

The direct cold-exposure trials are split. In a 1968 controlled rhinovirus challenge, prison volunteers were inoculated with virus and then either chilled in 4°C air or kept warm; there was no difference in infection rate or symptom severity. Cold exposure couldn't make a fresh viral challenge stick more often Douglas et al. 1968. Decades later, Johnson and Eccles ran a different design — 180 random adults asked to put bare feet in 10°C water for twenty minutes, with no inoculation. Within five days, 29 percent of the chilled group had cold symptoms versus about 6 percent of controls Johnson & Eccles 2005. The reconciliation that fits both: cold exposure can't create an infection from nothing, but in a population where some of the chilled volunteers were already carrying a quiet, sub-symptom virus, the peripheral chill tipped them over into a felt cold Eccles 2002.

Pulled together, the honest summary is the one the older review literature also landed on: cold exposure is a real but small modulator of respiratory infection risk, an order of magnitude smaller than viral exposure itself, and large enough to matter only because cold-season exposures are common and the interventions are nearly free Mourtzoukou & Falagas 2007.

Both grandmothers and doctors got it half wrong

Two errors, opposite sides of the same coin.

The folk error: cold weather catches you a cold. Wrong in its strong form. You can't catch a cold from a draft in an empty room. The virus has to be there.

The over-corrected medical-school answer from the second half of the twentieth century: cold weather has nothing to do with colds, only viruses do. Also wrong. Viral exposure is roughly constant year-round; infection rates absolutely are not. The difference is in your nose's defenses and how long an exhaled virion survives in the air around you — both of which run on temperature and humidity Moriyama et al. 2020, Huang et al. 2023.

The boring middle is the right answer. Viruses are the necessary cause, full stop. The air's hospitality and your nose's firepower modulate the per-exposure risk by enough to matter — and by enough that the cheap winter protocol below pays for itself in fewer sick weeks.

What you give up by keeping the wrong model

If you keep the grandmother model, you over-bundle and under-humidify. You avoid the winter walk — outdoors, dispersed, low-virus — and spend the long evenings indoors at 18 percent humidity breathing a coworker's exhaled virions hanging at chest height. The things you fear are mostly safe; the things you don't notice are doing the damage. The model and the actual lever point in opposite directions.

If you keep the over-corrected version — weather is irrelevant, only viruses matter — you assume nothing in your environment helps and roll the dice every December. The humidifier never gets bought; the vitamin D bottle never gets opened; the kid's classroom stays at 15 percent humidity all winter.

Across an adult lifetime of three to six respiratory infections a year, the difference between someone running the small protocol and someone treating cold season as fate works out to dozens of reclaimed sick days. The bigger cost is downstream, on the people you spread to. Influenza and pneumonia kill tens of thousands of older and immunocompromised people every winter in the United States alone; the bug you carry home in February is sometimes the one that lands on someone who can't shake it.

What to actually do

The levers fall out of the mechanisms. Almost all of them are cheap; almost all of them sit in your own house.

The single highest-yield adjustment is indoor humidity. Aim for 40 to 60 percent relative humidity. Below 40 and the aerosol-and-mucus mechanisms switch on against you Kudo et al. 2019, Yang & Marr 2011; above 60 and you start farming dust mites and mould instead.

What changes

Within one winter, you get fewer colds. Not zero — viruses still get through — but a real fraction less, and the ones that land tend to be four-day colds instead of ten-day ones. The defenses you kept intact attacked the virus before it had a chance to dig in.

The 3am dry-throat waking stops. The January nosebleed stops. The kid stops running a fountain from November to April. The cold floor of your winter — the days you brace for as inevitable — rises a few degrees.

Around year three of running this, the felt difference is mostly that you stopped bracing for cold season at all. You are not avoiding the cold; you are not surrendering to it either. You have a model that maps to what you can change, and most of what you can change you already did, in your own house, for under fifty dollars.

Adjacent topics this entry points at but doesn't cover end-to-end: vitamin D status and supplementation as its own check; influenza and COVID vaccination; nasal breathing and humidification as a year-round airway-health practice; indoor air quality and ventilation more broadly.

Substance + claimed effects

The folk belief: cold weather, chilled hands, wet hair, or a draft on the back of the neck causes the common cold. The substance under examination is the relationship between low-temperature exposure (ambient cold, cold air to the upper airway, peripheral chilling) and the incidence of upper respiratory tract viral infection — rhinoviruses, seasonal coronaviruses, respiratory syncytial virus (RSV), influenza, parainfluenza. The honest finding lives between the two slogans. The strict virological line — viruses cause colds; cold weather does not — is correct as far as it goes: without viral exposure, no infection. But it leaves out a real set of mechanisms by which low temperature and low humidity raise the probability that any given viral exposure becomes an established infection: temperature-dependent failure of the nasal epithelium's antiviral interferon response Foxman et al., PNAS 2015; suppression of nasal extracellular-vesicle "decoy" defense when inhaled air drops the vestibule temperature by ~5°C Huang et al., JACI 2023; dehydration of the airway mucus blanket and impairment of mucociliary clearance under low ambient humidity Kudo et al., PNAS 2019; longer airborne survival of small respiratory droplets at low absolute humidity Lowen et al., PLoS Pathog 2007; behavioural crowding indoors; reduced vitamin D synthesis under winter sunlight Martineau et al., BMJ 2017. The entry covers all of this and the consequences that follow: a small short-term health gain (fewer colds and flus per season when the reader maintains indoor humidity, hand hygiene, and vitamin D status), a small longevity contribution at the population level via reduced flu/pneumonia mortality, marginal effects on energy and sleep through fewer sick weeks and a less dry airway at night, modest cost (humidifier, hygrometer), modest effort (refilling, hand-washing during peak season).

Evidence by addressing question

mechanism

Temperature-dependent innate immunity in the upper airway. The nasal cavity sits cooler than core body temperature — typically 32–33°C in the anterior nares versus 37°C in the lower airways. Rhinoviruses, by far the largest contributor to common colds (~30–50% of cases in adults), evolved to exploit that thermal niche. Foxman et al. used mouse airway epithelial cells cultured at 33°C versus 37°C and showed that interferon-mediated antiviral signalling is markedly less effective at the cooler temperature: type I interferon induction is blunted, and apoptotic clearance of infected cells is slower; rhinovirus replicates much more efficiently at 33°C as a result Foxman et al., PNAS 2015. Inhaling cold air drops the nasal vestibule a further few degrees below baseline, deepening this defect.

Cold-induced suppression of nasal extracellular vesicles. Huang et al. recently characterised a previously unrecognised innate-immune mechanism in the human nose: nasal epithelial cells, on viral challenge, release a swarm of extracellular vesicles (EVs) that bind and neutralise virions before they reach receptors on the cell surface. The EVs carry decoy receptors and bactericidal/antimicrobial proteins. The authors then mapped the temperature dependence: when human subjects were exposed to 4.4°C ambient air for 15 minutes, intranasal temperature fell by ~5°C and EV secretion fell by ~42%, with a corresponding drop in in vitro antiviral activity against rhinovirus, coronavirus 229E, and H1N1 Huang et al., JACI 2023. This is the strongest direct mechanistic evidence to date that the folk wisdom — cold air itself weakens the nose's defenses, not just behaviour around cold weather — has biological substance.

The vasoconstriction hypothesis. Eccles proposed that cold-induced vasoconstriction in the nasal mucosa reduces local leukocyte delivery and creates a window in which a latent viral exposure can establish symptomatic infection Eccles, Acta Otolaryngol 2002. This frames the Johnson–Eccles foot-cooling trial (see evidence): the chill triggers the bloom in those already harbouring virus, rather than seeding a new infection from nothing.

Humidity and the mucus blanket. The airway is protected by a hydrated mucus layer that traps virions and clears them via ciliary beating. Low ambient humidity — endemic in heated indoor air in winter, where relative humidity often sits at 10–20% — desiccates the mucus, slows ciliary clearance, and impairs tissue repair after epithelial damage. Kudo et al. compared mice housed at 10% versus 50% relative humidity at 20°C; the low-humidity group had impaired mucociliary clearance, reduced tissue repair, decreased interferon-stimulated gene expression, and significantly higher influenza mortality Kudo et al., PNAS 2019.

Aerosol stability at low humidity. Respiratory droplets exhaled during breathing, talking, and coughing evaporate faster in dry air, leaving behind smaller residual particles that remain airborne for minutes to hours rather than seconds. Influenza virion infectivity in suspended aerosols collapses above ~50% relative humidity and stays high below ~40% Yang & Marr, PLoS ONE 2011. Aerosol-route transmission is a major contributor to respiratory virus spread; the small-particle physics of low humidity therefore directly enables transmission Tellier, J R Soc Interface 2009.

evidence

Direct cold-exposure trials are mixed but tilted toward "yes, a small effect." Douglas et al. ran a controlled rhinovirus challenge in 1968: prison volunteers were inoculated, then exposed to cold (4°C nude, or wet clothing in 26°C) or kept normothermic. There was no difference in infection rate or symptom severity — cold exposure failed to amplify a fresh viral challenge Douglas et al., NEJM 1968. Johnson & Eccles got the opposite finding decades later with a different design: 180 unselected adults randomised to immerse their bare feet in 10°C water for 20 minutes or to sit with feet in an empty bowl. Within 4–5 days, 13/90 (29%) of the chilled group reported common-cold symptoms versus 5/90 (5.6%) of controls Johnson & Eccles, Fam Pract 2005. The two findings reconcile through Eccles's vasoconstriction model: cold exposure doesn't create infection in a virgin host, but in a population where some subset is already shedding subclinical virus, a peripheral chill can shift the balance toward symptomatic disease. Mourtzoukou & Falagas reviewed the broader literature on cold and respiratory infection and concluded that the effect of cold exposure on respiratory infection rates is real but small relative to viral inoculum size and contact patterns Mourtzoukou & Falagas, Int J Tuberc Lung Dis 2007.

Seasonality is robustly real. Respiratory viral infections in temperate latitudes cluster in autumn and winter, with peaks for rhinoviruses in autumn and again in spring, influenza in midwinter, RSV in winter. The seasonality is multifactorial — humidity, temperature, behaviour, school terms, vitamin D — and the relative weights are still debated. Moriyama, Hugentobler & Iwasaki review the candidates and lean toward absolute humidity and ambient temperature as the dominant environmental drivers, with crowding and school transmission as the dominant behavioural drivers Moriyama et al., Annu Rev Virol 2020.

Humidity and influenza transmission — animal model. Lowen et al. infected guinea pigs at 5°C and 20°C across a humidity gradient (20–80% RH). Transmission to naive cage-mates was most efficient at low humidity and low temperature; at 30°C, transmission collapsed entirely Lowen et al., PLoS Pathog 2007. The result has held across replications and is one of the foundations of the absolute-humidity hypothesis of influenza seasonality.

Humidity as a population-level lever. A natural experiment in a Minnesota preschool — humidifying classrooms to ~45% RH during winter — reduced absolute viral counts on surfaces and in air, and tracked with lower flu-like illness, though the design was uncontrolled Reiman et al., PLoS ONE 2018.

Vitamin D — the most reliable winter-specific prophylactic. Martineau et al. pooled individual-participant data from 25 RCTs, 11,321 participants, of vitamin D supplementation versus placebo. Across the board the relative risk reduction for acute respiratory tract infection was modest (adjusted odds ratio 0.88, ~12% RR reduction; NNT ≈ 33). In participants with baseline deficiency (serum 25(OH)D < 25 nmol/L) the effect was much larger (NNT ≈ 4), and daily/weekly low-dose schedules out-performed high-dose bolus schedules Martineau et al., BMJ 2017. Winter is when temperate-latitude populations drop deficient, so this is the cleanest cold-season intervention with RCT-grade evidence.

protocol

The protocol-relevant levers fall out of the mechanisms. Indoor humidity at 40–60% RH dampens aerosol survival, keeps mucus clearance functioning, and is the single highest-yield environmental adjustment Yang & Marr, PLoS ONE 2011, Kudo et al., PNAS 2019. A 40–60% relative humidity target needs a hygrometer to verify; most heated winter homes sit at 15–25%. Above 60% the marginal antiviral gain is offset by mould/dust-mite risk. Ventilation and indoor crowding — opening windows for a few minutes during gatherings, avoiding poorly-ventilated dense indoor settings during local peak weeks — directly attack the aerosol-residence-time problem. Hand hygiene during the local peak season interrupts the fomite route (rhinovirus is particularly fomite-stable). Keeping the nose warm — scarf or buff over the nose in cold air — is the operationalisation of the Huang/Foxman mechanisms; modest effect, near-zero cost. Vitamin D 1,000–2,000 IU/day daily through the dark months for temperate-latitude adults, more if a serum 25(OH)D test shows deficiency Martineau et al., BMJ 2017.

misconceptions

Two opposite errors. The folk error: cold weather causes colds. The over-corrected medical-class error: cold weather has nothing to do with colds; only viral exposure matters. The literature supports neither. Viruses are the necessary cause; without exposure there is no infection. But viral exposure is constant year-round, while infection rates are wildly seasonal — the difference is the host's defenses and the air's hospitality to the virion, both of which are temperature- and humidity-dependent Moriyama et al., Annu Rev Virol 2020. The grandmother-was-half-right framing is the accurate one.

A subsidiary misconception: that wet hair, a draft on the neck, or "letting yourself get chilled" is a leading cause of colds. The trial evidence here is mixed; Johnson–Eccles showed an effect for foot cooling but only in a population where subclinical viral shedding was plausible Johnson & Eccles, Fam Pract 2005, while Douglas's controlled rhinovirus challenge in 1968 found no effect from soaking-wet or freezing exposures on infection rates in non-carriers Douglas et al., NEJM 1968. The strongest fair statement: peripheral chilling is a small modulator on top of an exposure that has to happen anyway.

practicalities

The humidity target is operationally cheap. A passive evaporative humidifier or warm-mist humidifier costs $30–80 USD and runs through winter on tap water; a hygrometer is $10–20. The genuine downsides: water-tank cleaning to avoid bacterial/mould aerosolisation, mineral dust from hard-water ultrasonic units, and over-humidifying past 60% RH which favours dust mites. Whole-home humidifiers tied to HVAC are $300–1,000 installed and remove the per-room hassle. Vitamin D supplementation at the doses studied is on the order of $10–20 per year. Hand hygiene and ventilation are free.

stakes

The cost of getting the model wrong runs in two directions. If the reader believes cold weather directly causes colds, they over-invest in keeping warm and under-invest in the actual transmission levers (humidity, ventilation, hand hygiene during local peaks); they may avoid outdoor cold exposure that is in fact a low-risk environment (transmission is overwhelmingly indoor); they reach for antibiotics or zinc or vitamin C with no real model of why they keep getting sick. If the reader believes cold weather has nothing to do with it — the over-corrected position — they ignore the humidity-mucus-aerosol axis and the keep-the-nose-warm guidance, both of which carry small but real per-exposure protection. Across a 30-year adult life with three to six respiratory infections per year on average, even a 10–20% per-season reduction integrates to dozens of fewer sick days, with the largest gains in winters spent around small children, in dense workplaces, or with vulnerable household members for whom the bug the reader brings home is much more than a sick week. At the population level, influenza and pneumonia cause tens of thousands of deaths per year in the US alone, concentrated in elderly and immunocompromised contacts, so the reader's transmission practices have a small but real second-party-mortality tail.

payoff

Within one winter, a reader who runs the humidity-ventilation-hygiene-vitamin-D protocol gets fewer respiratory infections — not zero, but a real fractional reduction — and the ones they do get tend to be shorter and milder, because the airway defenses they kept intact attack the virus faster. Side effects: less dry-throat in the morning, less nosebleed, less waking from a dry mouth. Over years, the reader stops surrendering winter to a feeling of inevitability and gets back the autonomy that comes with understanding a real causal chain.

out-of-scope

Adjacent topics that the entry signposts but does not cover end-to-end: vitamin D supplementation as its own entry; influenza vaccination as its own entry; nasal breathing and humidification as a year-round nasal-health topic; hand hygiene; indoor air quality and CO₂ as a separate substance.

The credibility range

Optimist case

Cold air directly and measurably suppresses the nose's antiviral defenses through at least two distinct mechanisms (Foxman's interferon-temperature dependence and Huang's EV suppression); low humidity directly enables aerosol-route transmission (Yang & Marr, Lowen); the Johnson–Eccles foot-cooling result shows that peripheral chilling alone can convert subclinical exposure into clinical infection; and Martineau's IPD meta-analysis shows vitamin D supplementation gives a population-scale ARI reduction. Read together, the folk wisdom about cold weather and colds was tracking real biology that the second half of the twentieth century dismissed prematurely. The Huang 2023 paper in particular reopens a question that "viruses cause colds, not weather" educators thought settled.

Skeptic case

No exposure, no infection — this is the bedrock. The Douglas 1968 controlled challenge showed cold exposure does not make a viral challenge stick more often. Johnson–Eccles is uncontrolled for prior viral shedding and reports symptoms by self-report (susceptible to expectancy effects in a study where the intervention is overt). Foxman's temperature dependence is established in mouse airway cells, not human population trials. Huang's EV finding is a single-paper result and quantifies an antiviral function whose downstream effect on infection rates in humans has not yet been measured. The Lowen humidity work is guinea pigs. Aggregated, the mechanistic evidence is suggestive and consistent but the human-outcome evidence is thin; behavioural factors (indoor crowding, school terms, holiday travel) likely dominate the seasonality signal in practice. The pragmatic skeptic position: yes there is a small environmental contribution, but the reader's biggest lever is still hand-washing and not being inhaled-on by sick coworkers.

Author's call

The honest landing is between the two slogans, but closer to the optimist side than the medical-school orthodoxy of the late twentieth century admitted. The mechanistic story (interferon-temperature dependence; EV swarm suppression at 5°C delta; aerosol residence time at low humidity; mucociliary clearance failure at low humidity) is now multi-paper, multi-mechanism, and converging. Population-level outcome data (Martineau on vitamin D; Moriyama et al.'s seasonality review) tracks the same directions. The Johnson–Eccles peripheral-chill result is the weakest link and probably an over-call as a clean "cold causes colds" finding, but it is consistent with the broader story when read through Eccles's vasoconstriction model. So the entry lands as: cold weather is a real per-exposure risk modulator, an order of magnitude smaller than viral exposure itself, and the reader who treats it as nothing is leaving a small but cheap intervention bundle on the table.

Stakeholder + incentive map

The "viruses cause colds, not weather" educators — public-health communicators and physicians who, for decades, have been correcting the folk model. The correction was scientifically grounded for its era but became its own oversimplification; the recent mechanism papers (Foxman, Huang) are now an awkward update for that camp.
The grandmother-was-right wellness camp — overstates the case; tends to push cold-avoidance, nasal rinses, layered scarves as silver bullets with stronger claims than the data supports. Aligned with the entry's direction but typically without the qualifying nuance.
Humidifier manufacturers — commercial interest in the humidity story; the underlying science is independent of the marketing, but reader-facing claims often inflate the per-unit health gain.
Vitamin-D-supplement industry — same: commercial interest in inflating the deficient-population finding to the general population, where the effect is much smaller.
Flu-vaccine and respiratory-vaccine programs — institutional incentive aligned with reducing respiratory infections but typically not engaged with the environmental side of the story.
HVAC and indoor-air-quality industries — newer commercial entrants pushing whole-home humidification and ventilation; mechanistic claims hold up better than supplement-side claims do.

Population variability

Children and households with young children have the highest baseline respiratory-infection rate (children average 6–8 colds per year). The relative gain from any environmental intervention is largest for them and for the adults exposed to them daily.
Elderly — higher mortality risk from flu and pneumonia; vitamin D deficiency is more common; humidity benefit is amplified because mucociliary clearance declines with age.
Vitamin-D-deficient populations (high latitudes, dark skin in temperate climates, indoor-bound) — Martineau's NNT for ARI prevention drops from ~33 in the general population to ~4 in deficient subgroups Martineau et al., BMJ 2017.
Asthma, COPD, immunocompromise — bigger stakes per infection; the entry's environmental levers apply but the primary substance is the underlying condition's protocol.
Tropical and equatorial readers — seasonality is muted but not absent; the rainy-season peak in many tropical regions follows humidity rather than temperature, complicating any simple cold-air story.
Latitude effects on vitamin D — readers above ~40° latitude essentially cannot synthesise vitamin D from sunlight between October and March.

Knowledge gaps

The single largest gap: Huang's 2023 EV mechanism is established in vitro and the temperature-of-nose drop confirmed in vivo, but no clinical trial has yet tested whether interventions on nasal warmth (nasal warming devices, scarves, breathing through a buff) reduce infection rates in field populations. A clean Foxman-Huang clinical RCT would be cheap and informative; it hasn't been done. The aerosol-route share versus droplet/fomite share of respiratory virus transmission is debated and varies by virus (rhinovirus is more fomite-stable; influenza and SARS-CoV-2 lean more aerosol). Humidity-intervention trials at scale (workplaces, schools) are sparse and mostly uncontrolled. The interaction between the host's vitamin D status and the temperature/humidity mechanisms above has not been mapped — they may be additive, multiplicative, or substitutable, and current evidence does not separate them. Finally, the relative weighting of behavioural drivers of winter seasonality (indoor crowding, school terms) versus environmental drivers (humidity, cold air on nose) is still genuinely open Moriyama et al., Annu Rev Virol 2020.

Scope and the brief. The input description named seasonality, indoor crowding, humidity, nasal immune defenses at lower temperatures, and the exposure-versus-infection distinction. All five are covered end to end — seasonality and crowding in evidence; humidity in mechanism, evidence, and protocol; nasal immunity in mechanism; the exposure-vs-infection distinction is the spine of misconceptions. No silent narrowing.

Category call. Considered three: mindset (the entry is fundamentally a mental-model calibration), home (the actionable levers are environmental — humidifier, ventilation), and breathing. Landed on breathing because the substance itself is respiratory immune defense; the home-environment levers fall out of that substrate rather than the other way around. A reader looking for "why I keep getting sick" finds it under airway, not mindset.

Action and cadence. Chose know over do: the entry's primary work is calibrating two opposite-direction misconceptions; the protocol is real but small and bolted onto the literacy. Cadence as-needed rather than daily; the humidifier and vitamin D are daily but the entry isn't a daily-action entry — it's read once and lived with through cold season.

Evidence score 3 was the hardest call. The mechanism literature is multi-paper and converging (Foxman 2015, Huang 2023, Kudo 2019, Lowen 2007, Yang & Marr 2011); the vitamin D arm has an IPD meta-analysis (Martineau 2017); seasonality data is overwhelming. But field-trial outcome data for the nasal-warming / scarf-the-nose arm is essentially absent — the strongest direct mechanism paper, Huang 2023, characterises an antiviral function that has not yet been measured against infection rates in humans. Held at 3 because the mechanism-to-outcome inference is unusually clean here and the vitamin D arm carries the strong-evidence weight.

Controversy score 2. The educator consensus "viruses cause colds, not weather" sits awkwardly against the new mechanism papers. It is not a paradigm fight — the disagreement is at the margins of an otherwise stable picture — but the entry deliberately names both errors rather than picking a side, and the reader is told this is an active update.

Dream narrative written despite score 32. Below the 40-point mandate, but the relief lever (stop surrendering winter to a model that points in the wrong direction) is honestly strong and the dek leans on it.

Johnson & Eccles 2005 kept despite methodological weakness. Self-report endpoint, unblinded intervention, no measurement of pre-existing viral shedding. Kept because the Eccles vasoconstriction model reconciles it with the otherwise-null Douglas 1968 challenge, and dropping it would leave the only direct human cold-exposure data unrepresented. Flagged in the article as a contested result.

Excluded — different substance.

Cold-water immersion / deliberate cold exposure for immune training (Wim Hof, cold plunge). Different substance, different evidence base, deserves its own entry. Adjacent but not subsumable.
Zinc lozenges, vitamin C, echinacea, elderberry, NAC. Each is a separate supplement entry; this one would inflate beyond its scope.
Antibiotics. Wrong drug class for viral URIs; the under-prescribing case is its own decide-action entry.
Nasal saline / Neti pot. Likely a separate entry under nasal breathing or airway hygiene.

Future-link candidates. Once they exist, this entry should cross-link to: vitamin D supplementation (it's the only outcome-trial-grade winter intervention named); influenza vaccination; nasal breathing; indoor air quality and ventilation; humidifier-as-device (cleaning, hard-water risks). Listed in out-of-scope already; wire in when those entries land.

Separate-entry candidates raised by the writing. The humidifier itself is a real candidate — device choice, cleaning protocol, ultrasonic-vs-evaporative tradeoffs deserve more than a paragraph. Vitamin D status testing is another. Both flagged for the backlog.