The nose is part of the remarkable air-conditioning system. The temperature of the air we breathe in is usually lower than our body temperature, and is not fully saturated with moisture. So the higher the air temperature, the higher the water content. We need the inspired air to be fully saturated so that when it comes into contact with the wet mucous membrane, it does not dry out.

It is not just this lining that needs to be kept moist, but also the delicate lining of the air sacs in the lungs. The role of the nose is also to warm inspired air, which is done by the blood supply in the nose. The tiny hairs in the nose not only move the air along, but also act as a filtering device to trap particles of debris and to waft germs out. When you breathe through your mouth, the air conditioning system is cut out and your throat can become dry and sore, as it is not designed to receive air direct from outside.

When mucus from the back of the nose drips down into the pharynx, this is referred to as post-nasal drip. This is perfectly normal and we do it. Although we are not usually aware of it except after a cold. Rather than coughing to clear it, use steam inhalations, drink plenty of fluids and don't be afraid to swallow it: it is quite harmless.

It is important that we all have mucus as it serves many essential function in the body. The under-production of it (sometimes caused by misusing remedies or medication can cause problems. Or if it dries out, then it becomes sticky and no longer serves the purpose of being a lubricant. Germs can then multiply in it. This is what happens during a cold. Use of medication such as antihistamines, tranquillisers and decongestants also have dryness as a side effect.