Pollution or what is it dangerous ???!

So I read on here that someone visited London and said it was soo polluted that at the end of the day if you spit you can find black stuff I didnt believe this till I did it everyday for a week and every day I had it!. Bu when I stay in I didnt have it!. I use the London Underground 5 days a week so Im guessing it must be from there aint this a health hazard its bad enough being packed next too so many people on rush hour or when arsenal play now this!. so will this cause health problems or are there any known ones
It must be the underground because the tunnels are dirt black Www@Answer-Health@Com

---

---