Who is at risk of NFCI?
Risk factors may be grouped according to type: environmental, mechanical, physiological, psychological and medical [1]
Environmental
Cold
It goes without saying that the colder the environment the more likely it is for extremity cooling to pose a problem, but there is no known dose-response curve [2] and therefore no minimum or maximum thresholds for ambient temperature or exposure duration.
Wind
Increases the rate of cooling through enhanced convective heat loss.
Wet
Cooling is increased by damp or wet conditions, through a combined effect of increased heat loss through conduction (the thermal conductivity of water is 25 times that of air [3]) and evaporation.
Cold objects
Handling cold objects, particularly metal or other highly conductive materials, causes more rapid cooling through enhanced conductive heat loss [4].
Mechanical
Immobility
Staying still for a prolonged period reduces heat production, and also reduces blood flow to the hands and feet, making them more susceptible to NFCI.
Clothing
Being generally cold increases the risk of NFCI [5]. In a cold environment, wearing clothing with insufficient insulation will result in heat loss and the body cooling - particularly the hands and feet. Clothing insulation will be reduced if it gets wet (from sweat, rain, water immersion - which also worsen evaporative cooling). Wind enhances convective and evaporative cooling if an outer windproof layer is not worn. Wicking layers should be worn and the neck and head should also be covered. Wet clothing will further cool the skin due to evaporation.
Waterproof boots, gaiters and warm socks should be worn in wet and cold conditions but should not be too tight as this will reduce blood circulation. Multi-layered glove systems may be required in cold conditions, such that the hands are not exposed even when doing fine tasks, and the outer layer should preferably be a waterproof and windproof mitt.
Physiological
Constitutional cold sensitivity
Response to cold is individual; people of similar age and BMI can react very differently. This ‘constitutional cold sensitivity’ is usually well recognised by the person, who is aware that they feel cold sooner than others [6] and frequently has family members who respond similarly; these people probably have a higher risk of developing NFCI.
Race
Individuals of African and Caribbean race appear to be at greater risk of cold injury [5]. This may be due to a slower rewarming rate after cold exposure [7] possibly due to reduced reactivity of the blood vessels in the fingers and toes [8].
Sex
Females tend to have colder hands and feet and cool faster than men [9] and are more likely to have Raynaud's Phenomenon. Therefore, they may be at greater risk of NFCI. However, due to the low numbers of women in the military (where most epidemiological research on NFCI has been conducted), no studies have confirmed this.
Age
With ageing, thermal control in the cold gradually becomes less efficient. This is thought to be due to a combined effect of not sensing the cold as quickly [10], reduced vasoconstriction in the hands and feet [11] and reduced ability to produce heat from metabolism [12] once the cold is felt. Older individuals may also have a reduced cold-induced vasodilator (CIVD) response [13]. However, these effects are not significant until about the age of 50 years.
Smoking
Smoking reduces the blood flow to the fingers [14] and therefore could put an individual at greater risk of NFCI. However, the evidence for smoking increasing the risk of NFCI is weak [5].
Fatigue/reduced calorie intake
Fatigue and reduced calorie intake may impair the ability to produce heat through exercise or shivering. This will result in a fall in core temperature and cause greater vasoconstriction in the hands and feet making them more susceptible to cold injuries.
Previous cold injury
Individuals who have had a previous cold injury may be at greater risk [15], particularly if they have residual cold sensitivity or hyperhidrosis (excessive sweating in the injured area).
Psychological
Inexperience
Those new to the military are at greater risk of NFCI [5]. This is probably due to a combination of factors: emotional stress, which invokes the release of the vasoconstrictor adrenaline; poor personal administration and equipment use; and lack of experience of cold exposure and its related symptoms. It is likely that inexperienced civilians would also be at increased risk for the same reasons.
Medical
Raynaud’s Phenomenon
There is a good scientific rationale for individuals with Raynaud’s Phenomenon (RP) to have a higher risk of NFCI as RP causes a marked reduction in blood flow to the hands and/or feet with cold exposure and anxiety [16]. However, no research studies have been conducted to support this.
Acrocyanosis and livedo reticularis are less well recognised variants of primary RP that, due to their association with reduced skin blood flow in response to cold [17] should also be regarded as risk factors for NFCI.
Hand Arm Vibration Syndrome
There is a good scientific rationale for individuals with Hand Arm Vibration Syndrome (HAVS) to have a higher risk of NFCI as HAVS causes a marked reduction in blood flow to the hands [18]. However, no research studies have been conducted to support this.