Being able to breathe is a big issue and unfortunately the pollution levels in our cities is so high that going out for a run or cycling is more harmful than the health benefits.
So to counter this, there are a few filters available.
Pollution China has a nice article on how to choose the right facemask from Respro.
This is the sport mask, but there are many different products offering different protections and comforts.
Respro: Sports & Leisure – Cycling – Sportsta Mask.
Sanspira also offers nasal filters which are much much smaller but obviously don’t offer much in the way of filtration when breathing through the mouth. here