You used to see them everywhere
Hanging on the wall.
On buses and in shelters.
But now there's none at all.
I never wanted then to spit.
Perhaps that was just me.
But now and then I feel the need
With this C..O..P..D
I know it's not acceptable,
When your throat begins to itch.
Unless, of course, you happen to be
On a football pitch. (Uck!)
Spitting is disgusting but sometimes necessary as you say Don. Xxx
That's why people carry tissues, though I can see the problem there for footballers. 
People in London spit anywhere..great TB carrier..
"When your throat begins to itch." ...When that happens I suck a Fisherman's Friend 'sweet' and the itching goes away. Don't go anywhere without a packet of Fisherman's Friend.
The signs originated in Victorian times when postmen used to drag their bags along the ground. The bags picked up TB spores from spittle left by TB sufferers and when it dried out in the mail rooms it spread to the workers. The problem was reduced by arranging for the bags to be carried and a law against spitting (which I suspect still exists, but not much enforced). Of course, TB cases reduced with vaccination, but are now on the rise from people who have not been immunised. However, those of us with COPD and/or asthma don't necessarily have TB, so spitting is less of an issue (so long as it's done discretely and in an area unlikely to be disturbed). Although the mucous is harmless if swallowed, I'm not sure whether the body can reabsorb it, so it might be advisable to get rid of it if at all possible.
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