Years ago, I asked the old man (who knows such things) why we even have snot:
Well, it is a long story, but in essence, this is how it goes: the nose is the air contioning unit for the lungs. It works a bit like a Dyson vacuum cleaner. When you breath in, the nose spins the air into a spiral as it goes down the nose. All the bits of dust, earwigs, germs etc get spun by centrifugal force to the edge and become stuck to the mucus lining the nose. The mucus, with its trapped bits, is moved from the front to the back of the nose by thousands of tiny whiskers, which are beating together all the time. The whole thing behaving like moving fly-paper. The central core of clean air goes down to the lungs, while the mucus goes down to the stomach. There, the bugs and bits are destroyed, and the mucus broken down into glucose, and transfered back to the nose by the blood, where it is turned back to mucus again.Even sitting out in the sun for hours on hasn't sorted it out. Bleurgh. Was picnicing at posh opera, thanks to some well-connected chums. Lovely day - though getting home turned into a bit of a faff, and only possible due to great kindness of other people.
Sometimes the system doesn't work. Allergy makes little white blood cells diffuse into the mucus, and this can make it turn yellow or even pale green. The cells are not really white, just whiter than the red ones. Some germs excrete coloured dyes, mostly green, and these can poison the little whiskers, so they won't beat. The mucus doesn't move along; the water in it evaporates, and it gets very sticky and snotty. If it gets snotty enough, germs can even grow in it. So it all get a bit complicated, especially as acid in the air, and other pollutions, can damage the little whiskers too.
So now you know! A potent source of acid in the air is from exhaust fumes of cars and lorries and so on. There is a hole in the ozone layer over Switzerland. If you are south of about Birmingham, you are under this hole. The hole allows ultra-violet light through, and this light reacts with the exhaust fumes to make them more toxic to the nose. So London is a good place to live!
Somewhat to my surprise, though, I managed to tie proper bow-ties. Blimey. And despite the raw, red nose, the Doctor seemed to approve of the outfit. She, of course, looked quite brilliant.
Back to work yesterday after some days of due to head being full of snot. Almost through all the Christmas stories. Oh, and I'm being interviewed on Friday about UNIT. Then off to a stag weeked for, by complete coincidence, one of the cast members.