The Rite Wing TechnoPagen points out that these days, you are actually advised to keep your towel with you!
In an interview with counterterrorism expert Joval Aviv, he says the following:
Since mass transportation is the next attack, when you travel to work have with you, a bottle of water, a small towel and a flashlight. What happened in London is exactly a point to look at. Those people who were close to the bombs died, then others were injured or died from inhaling the toxic fumes or getting trampled. The reason you take a bottle of water and a towel is that if you wet the towel and put it over your face, you can protect yourself against the fumes and get yourself out of there.
So next time you're out, be a Froopy Dude (or Dudette)!