Why Do Chilblains Come on Days of Big Temperature Swings, Not in Deep Winter?
― Fingertip Vessels Narrow Fast but Reopen Slowly
Chilblains are not caused by your fingers freezing. Blood vessels that have narrowed in the cold fail to fully reopen on the "outflow" side when you warm up. Blood pools in the meantime and seeps out into the tissue. That is why they are most likely in the changing seasons, when you swing between chilled and warmed again and again.
Your breath is white in the morning, yet by noon it is so warm you don't need a jacket. A few days of early winter weather like that go by.
One night, after a bath, you slip your feet under the kotatsu (a heated table). Your toes start to itch. You look down, and your little toe is swollen and reddish purple.
It isn't even the depths of a snowy winter. Why? And the itching comes not when you are cold, but when you warm up.
There are two main reasons
To keep heat in, the body narrows the vessels at the ends of your limbs. If a vessel's width is halved, the blood flowing through it drops to one sixteenth.
The vessels that send blood in open quickly, but the vessels that carry it back open slowly. Blood with nowhere to go pools and seeps out through the vessel walls.
In other words, chilblains come not from the cold itself but from the switch from "cooling" to "warming". Being tossed about by rising and falling temperatures many times in a day is the trigger.
A slightly narrower vessel cuts blood flow sharply
Your fingertips, earlobes and nose tip are far from the centre of the body and stick out into the air. They are the places where heat escapes most easily. So when the body senses cold, it squeezes the thin vessels near the skin and keeps warm blood deep inside.
This squeezing is more powerful than you might expect. The amount of liquid flowing through a tube is said to be proportional to the fourth power of its radius. Halve the radius, and the flow is not halved but cut to one sixteenth. Try changing the vessel width with the slider in Figure 1.
For the body, this is the right response to protect heat. The trouble comes afterwards, when you warm up.
When you warm up, only the inflow opens first
Roughly speaking, the blood route in a fingertip has two parts: an "inflow" and an "outflow". The thin arteries that bring blood from the heart are the inflow. The veins that return blood to the heart are the outflow.
When a chilled finger is suddenly warmed, the inflow arteries open quickly. The outflow side, however, is thought to stay narrowed for a while longer. Look at Figure 2. More blood comes in, but the way out stays narrow. Blood pools in the fingertip, which swells and turns reddish purple.
When blood pools and the pressure inside the vessel rises, water from the blood seeps through the vessel wall. This is the swelling. In addition, inflammation is thought to arise around vessels that have been chilled repeatedly, causing itching and pain.
That is why a single cold day rarely does it. In Japan, chilblains are said to be common when the lowest temperature is around 4–5℃ and the daily temperature swing exceeds 10℃. This is why they increase in early winter and early spring rather than in freezing mid-winter.
Water carries heat away far faster than air. If you stay in socks damp with sweat, or in gloves soaked from playing in the snow, your fingertips get strongly chilled even when the air isn't that cold. One reason chilblains are common on children's feet is that they get wet easily playing outside.
Frostbite is damage from skin and flesh actually freezing. Chilblains are a disturbance of blood flow that happens at temperatures that do not freeze. However, if the purple turns blackish, or blisters break and do not heal, see a dermatologist.
So what should we do?
Once you know the mechanism, prevention is clear. There are three points: keep warm, don't stay wet, and don't warm up suddenly. Putting cold feet straight into hot water or next to a heater makes the mismatch between inflow and outflow the largest. Rewarm slowly in lukewarm water.
Summary
Chilblains are not a mark of failing to endure the cold. They are the mark left on fingertips that are forced, again and again, to narrow their vessels with each chill and reopen them with each warming, until they can't keep up with the lag on the outflow side.
What makes chilblains is not the cold, but the back-and-forth between cold and warmth.
Warming up slowly is the best prevention.
How cold stops your fingers working is explained in "Why can't you fasten a single button when your hands are numb with cold?", and the danger of even colder conditions in "Why does hypothermia happen even on days that aren't that cold?"
- Dip the fingertips of one hand in cold water (tap water with a few ice cubes) for about 30 seconds. If it is too cold and hurts, stop right away.
- Take them out and compare the nail colour with your other hand. The nails of the chilled fingers should look whitish, drained of blood.
- Wait a few minutes at room temperature. The nail colour gradually regains its redness, and for a while may even be redder than the other hand.
The whitening is the vessels squeezing; the return of redness is the vessels reopening. Do not try this if you already have chilblains or if your fingers change colour strongly.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school biology or physics
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity specialist courses not taught in high school (physiology, fluid mechanics, dermatology)
- ResearchNot yet taught as settled fact even at university; what researchers are investigating now
Junior highTerms: this phenomenon has names
- Chilblains (tōsō): the medical name for chilblains. A condition in which repeated cold, not freezing, and rewarming makes the ends of the limbs, ears and nose swell reddish purple, with itching and pain.
- Vasoconstriction: when the thin vessels of the skin narrow under the action of the autonomic nervous system on sensing cold. It reduces heat escaping from the body.
- Congestion: blood that does not drain away and stays in one place. The reddish purple of chilblains comes from this.
Junior highHigh schoolChecking with a formula: how much does flow drop when a vessel is half as wide?
The amount of liquid flowing slowly through a thin tube is proportional to the fourth power of the tube's radius. Assuming this roughly applies to blood too, we calculate the flow when a fingertip vessel is squeezed.
| In symbols | Q ＝ π × r⁴ × ΔP ÷ ( 8 × η × L ) |
| In words | Flow ＝ pi × radius to the 4th power × pressure difference between the ends ÷ (8 × stickiness of the liquid × tube length) |
| Where it comes from | It follows from the condition that the push from the pressure difference between the ends balances the drag of the liquid's stickiness (viscosity) against the tube wall (the Hagen–Poiseuille equation) |
| Symbol Q | Flow (volume per second) |
| Symbol r | Tube radius. Here we use the ratio to normal, with normal = 1 |
| Symbols ΔP, η, L | Pressure difference, stickiness, length. Treated as unchanged before and after squeezing |
| Radius of a squeezed vessel | 0.5 times normal (an example for explanation) |
| Radius of a slightly squeezed vessel | 0.8 times normal (an example for explanation) |
| Radius 0.5 times: square first | 0.5 × 0.5 ＝ 0.25 |
| Square again for the 4th power | 0.25 × 0.25 ＝ 0.0625 |
| What fraction of normal | 1 ÷ 0.0625 ＝ 16 |
| Radius 0.8 times: square first | 0.8 × 0.8 ＝ 0.64 |
| Square again for the 4th power | 0.64 × 0.64 ≒ 0.41 |
| Example of a daily temperature swing (high 12℃, low 2℃) | 12 − 2 ＝ 10 |
Squeezing the radius by just 20% drops the flow to about 40%. Halve it and the flow is one sixteenth. A small change in width changes the heat reaching the fingertip a great deal. That is exactly why even a slight lag in the outflow side's recovery creates a big gap between the blood going in and the blood going out, and blood pools.
High schoolHigh school+Protecting body heat, and its side effect
High schoolThe width of skin vessels is regulated by the sympathetic nerves, part of the autonomic nervous system. In the cold the sympathetic nerves act and squeeze the vessels; in the heat they relax and let heat escape. It is a classic example of homeostasis in high-school biology.
High school+Fingertips have shortcuts called arteriovenous anastomoses, which connect arteries and veins directly without passing through capillaries. They close in the cold and open wide when warm. Opening and closing these shortcuts is thought to swing fingertip blood flow widely.
UniversityMicrocirculation and the cold-induced vascular response
In university physiology, blood flow in thin vessels is treated as "microcirculation". The basis for flow in thin tubes is the Hagen–Poiseuille equation, and the fourth-power law comes from it. But because blood contains red blood cells, corrections are needed in very thin vessels, such as the apparent stickiness changing (the Fåhræus–Lindqvist effect). It is also known that when a finger is kept in cold water, vessels that have narrowed open and close periodically, the cold-induced vasodilation (the "hunting reaction"). In dermatology, chilblains (tōsō) are explained as an abnormal vascular reaction to cold stimulation, together with inflammation.
📖 Derivation of the formula and beyond: Hagen–Poiseuille flow (Wikipedia, Japanese) ／ Chilblains (Wikipedia, Japanese)
ResearchWhat is still not clear
- Why do some people get them and others not, in the same cold? Constitution and genetics have been pointed to, but it is not well understood which factors matter, or how much.
- How does the inflammation start? Whether pooled blood comes first or inflammation of the vessel wall comes first, there are various theories on the fine order of events.
- The role of cold-induced vasodilation. Whether the periodic opening of chilled finger vessels prevents chilblains, or has little to do with them, is still debated.
So the content of this article, too, is "an explanation of what is understood so far". Read the idea of a mismatch in recovery speed between inflow and outflow as one commonly used way of thinking.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Year 2: "Blood circulation" | Arteries and veins, inflow and outflow |
| High school | Biology: "Autonomic nerves and homeostasis" | How skin vessels are squeezed in the cold |
| High school+ | Advanced biology: "Thermoregulation" | Fingertip arteriovenous anastomoses |
| University | Physiology, fluid mechanics, dermatology | Hagen–Poiseuille equation, cold-induced vasodilation, chilblains |
| Research | Vascular biology, dermatology | Individual differences, how inflammation starts |
| ― | Link to daily life | Change wet socks; rewarm cold feet slowly in lukewarm water |
- Wikipedia, "凍瘡" (Chilblains)
- Wikipedia, "ハーゲン・ポアズイユ流れ" (Hagen–Poiseuille flow)
- 本郷利憲・廣重力ほか監修『標準生理学』医学書院 (Hongo Toshinori, Hiroshige Tsutomu et al., eds., "Standard Physiology", Igaku-Shoin; chapters on thermoregulation and circulation)
- 清水宏『あたらしい皮膚科学』中山書店 (Shimizu Hiroshi, "New Dermatology", Nakayama Shoten; section on chilblains)
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. If symptoms are severe, last a long time, or the colour turns blackish, do not decide for yourself; see a medical institution.