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Getting lippyAs soon as the temperature falls below about 5 °C I have to avoid smiling because, if I do, my lips crack and bleed. Why does this happen and am I the only person to suffer in this way?

n Cold winter air cannot hold much moisture, so when it is warmed indoors to a comfortable temperature it will be drier than summer air of comparable warmth. This causes the skin to dry out more readily, rendering it less elastic and more prone to chapping.

Skin also expands and contracts with rising and falling temperature. This means the greater difference between indoor and outdoor temperatures in winter increases the stress placed on the skin as you go in and outside.

Lips are particularly prone to chapping because they lack the oil glands present on the rest of the skin whose sebum acts as a natural barrier to desiccation and helps to keep it supple. Repeated wetting of the lips through licking only makes things worse by stepping up evaporative cooling.

Lips are also frequently flexed in the course of eating, talking and changing expressions, and, in an already dried state, acidic and salty foods only inflame them more. Smiling provides the final insult because it pulls the corners of the mouth considerably, stretching lips to breaking point.

Why 5 °C should be so critical in

your case may require a little speculation, but the most likely explanation is that with the encroaching winter your skin has already dried out somewhat, and that as the biting air chills towards zero the last of any moisture it is holding condenses as frost.

Be reassured that you are far from alone in this, and the problem is relatively easy to remedy. Start applying lip balm or moisturiser regularly before the winter really sets in, and aim to wear a scarf that covers your mouth.

It is the same dry winter air that aggravates sinus headaches and raw coughs. Some remedies are simple – many householders find that humidifiers hooked over their radiators or having pot plants in the home provide relief.

Even an underlying medical condition should be readily treatable. It could just be that your lifestyle exposes you more than other people to the various factors. Perhaps they are the ones who need to get out more.Len WinokurLeeds, UK

Mole huntI have a mole out of which hairs grow much longer than in the surrounding skin. Why do some moles promote longer hair growth?

n There is more to hair growth than meets the eye. An eyelash, for example, is a marvel of programmed curvature, texture, colour, thickness, length and

persistence. The hairs on a porcupine are a microscopic and macroscopic tour de force. Even the growth of simple human body hair requires elaborate control of tissues in the papilla at the base of the follicle from which any hair sprouts. Such control demands interaction with surrounding cells and the body’s hormones.

Your long mole hair probably grows from papillae that originally produced fine, short vellus hair or humdrum body hair. The mole tissue grew from skin cells, mainly melanocytes, whose own growth controls had been disrupted by genetic or epigenetic changes to their DNA. They are now less inhibited and grow according to a

programme differing from those of surrounding tissues, so their cells’ textures and physiological products affect the growth of their neighbouring cells in turn. In particular, they can disrupt the activity of papillae, making them produce hairs unchecked. These hairs are often thicker and more simply structured than normal hair.

By contrast, a normal hair stops growing at shorter intervals, pauses for a suitable time, and is then shed after a few months or years of service. Jon RichfieldSomerset West, South Africa

This week’s questionsDairy DivisionSometimes I freeze milk. If I defrost it within two or three weeks of freezing it is fine. But if I defrost it after two or three months, it separates into a watery part and a thicker, white part. Why? How does it change during the time that it is frozen and apparently inert?Peter JonesUxbridge, Middlesex, UK

TasTe revelaTionThe first time I tried an olive I disliked it intensely. The same thing happened when I first drank wine. Now I love both. What happens to our sense of taste to allow us to start liking such foodstuffs? And, just as intriguingly, what makes us persevere with them? Tobias MontagueLondon, UK

WeiGhT of expecTaTionWhy, upon waking in the morning, am I usually rather desperate to defecate within about 10 minutes of rising?Ally PortmanManchester, UK

aGe MarkerElderly people sometimes develop what are known as liver spots, which are darker areas or blemishes on their skin. Why, and what causes them?Terry AndersonCarlisle, Cumbria, UK

“Mole tissue grows from cells whose growth-controls have been disrupted by Dna changes”

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