Table 3.

Cold air laboratory studies.

Reference	Study Sample	Habituation Length	Habituation Temperature	Cold Testing Procedure	Results/Findings
Kreider [69]	5 M Soldiers	Continuous 14 days	15.6°C (RH 40–50%, wind <1mph); wearing only shorts, with sheet and blanket at night	Longitudinal passive observation	Nocturnal Trec ↓ on the later cold days: Nocturnal toe temperatures were 15°C ↑ on the later cold days
Davis [68]	10 M maximally-acclimatized; 6 M minimally-acclimatized	8hrs daily for 31 days	11.8°C, wearing only shorts	Longitudinal passive observation; measurements made with subject nude during a 2 hr length of cold room exposure	By the 14th day, shivering in both groups ↓; Metabolic heat production ↔ in maximally-acclimated group but ↓ in the minimally-acclimated; in both groups, Trec ↓ after the 31 day exposure; Extremity temperatures ↔ in maximally-acclimated and slightly ↓ in the minimally-acclimated group
Keatinge [142]	14 M: 5 M passive cold exposure, 5 M warm room exposure, 4 M performed physical work in warm room	7.5 hrs daily for 19 days	6°C, air movement of 30 cm/s, wearing shorts, socks, and boots	Measurements taken the first and last day with cold exposure (6°C)	Physical activity group had ↓ early metabolic response to cold and maintained ↑ forearm Tsk after physical training; Passive cold exposure group had ↑ in the early daily metabolic response with a ↓ in metabolic rate at the end of each day, ↑ rate of Trec decline across daily exposure, ↔ Tsk, Trec, or intramuscular temperature; BMR unchanged in either group
Bruck [66]	14 M	1 hr exposure 4–7x within 2 weeks	Ambient temperature decreased from 28°C to 5°C to −5°C, wearing a bathing suit in a resting position	Longitudinal passive observation throughout repeated exposures	In 2/3 of subjects, metabolic heat production and shivering threshold occurred at ↓ Tb and Tes following repeated exposure, ↓ thermal discomfort and cold sensation, ↔ basal metabolic rate or Tsk
Mathew [120]	15 M soldiers	4 hr exposure to cold air daily for 21 days	10°C, wearing only shorts	Standard cold test (10°C air for 2 hrs) at days 1, 6, 11, 16, and 21	By day 21, ↑ RMR, smaller ↓ in Tb, ↓ shivering, ↑ CIVD and thermoregulatory efficiency, less rise in BP and HR during cold pressor response
Silami-Garcia [71]	10 F; 5 F cold-exposed, 5 F CON	Cold exposed: 10x for 1 hr within 2 weeks; CON: 2x within 18 days (for response measurement)	10°C air, clothing unknown	Measurements taken the first and last exposures (10°C air)	After ~5 exposures, cold-exposed women ↑ time to onset of shivering and ↓ metabolic heat production, ↔ Tsk, Tb, Trec, or big toe temperature
Armstrong [143]	4 F	10 days of daily cold air exposure	22°C for 45 min + 4°C for 45 min; wearing t-shirt, shorts, and cotton socks	Longitudinal passive observation: RMR measurements taken daily throughout exposures	During cold air exposure, RMR peaked at 31% VO2peak by the 5th min in Day 1, peak RMR on day 5 was ↓ (24%) and persisted through days 8 and 12; steady state RMR followed a similar ↓ trend beginning at day 5
Hesslink [8]	16 M; 8 M triiodothyronine supplementation, 8 M placebo	80 total (10x/week) 30 min exposures	4.4°C air, wearing shirt, shorts, socks	SCAT in basal conditions in January and again in March	↔ BMR, Tre; metabolic heat production, mean arterial pressure, and norepinephrine ↓ for all subjects in March, maintenance of T4 and TSH is not essential for habituation
Leppaluoto [107]	6 M	2 h daily for 11 days	10°C air, air velocity <0.2 m/s, humidity of 204 g/m3, only wearing shorts	Longitudinal passive observation: measurements taken daily for 11 days	↓ general cold sensations and those of hand and foot after the first exposure that remained throughout; ↑ Tsk and ↓ SBP after 4–6 exposures that disappeared by experiment end; forearm Tsk specifically ↑ after 4–6 exposures and remained to some extent throughout, ↓ NE response on days 5 and 10, ↔ Tre
Makinen [7]	10 M	2 h daily for 10 successive days	10°C air, air velocity <0.2 m/s, 50% RH (vs. 25°C air control), wearing shorts, socks, athletic shoes	Longitudinal passive observation: measurements taken days 1 and 10	With repeated 10°C exposure, ↑ Tsk and ↓ NE (24%); ↑ high frequency HRV power; ↓ increase in HR and blood pressure (specifically ↓ DBP) during handgrip testing
Park [144]	8 M	2 h of morning cold exposure + 2 h afternoon running/rest in the heat for 14 consecutive days	Cold: 10°C 40% RH with 0.31 clo Heat: 30° 60% RH with 0.28 clo	Pre- and Post-exposure program: cold tolerance test (10°C, 40%RH) with 0.21 clo – 60 min passive chair sitting in the cold	↓ SBP, DBP, MAP post exposure; ↔ Tsk or metabolic heat production; ↓ in thermal sensation but ↔ thermal comfort