Does your helmet stink because of perspiration, and of other things that you apply on your head? Is the inner cushion of your helmet dirty? Do you believe that wearing helmet is making your hair fall in increasing numbers or damaging your hair in some way? Do you get troubled by the dust and pollution in spite of wearing a helmet while riding your bike?
If you answer any of these questions in the affirmative, read on. I've spent considerable time figuring out solutions to these problems. One solution, I see most motorcyclists adopt, in the absence of bandanas, is wrap their skulls and faces (covering the mouth and nose) with handkerchiefs - under their helmets. I myself use two scarves every time I ride to my office or any other place which takes a little time reaching - one scarf I wrap around my skull and and the other I wrap around the lower part of my face. Done daily and frequently, it becomes quite a chore - rather tedious.
So here's a better alternative; use balaclavas. What are balaclavas? Very similar to monkey caps used during winters in India. But balaclavas have a different origin (you can read more here). You might've also seen robbers shown in movies wearing such head gear :).
For a long time, I've searched for this on google typing face mask or bike mask, but couldn't quite find the right stuff and the right place to buy. Today luckily, while again I typed the same words, I noticed the word balaclava in the results and finally found the right head gear and perhaps the right place to buy it too.
It will not only save you from dust and pollution and sunlight exposure, it's easy to use too - no more knotting bandanas every now and then. It might also help reduce some helmet-related hair problems. Very important to enjoy biking without getting bald. Hair matters :).
Balaclavas come in all sorts of material - stretchable, woollen, cotton, lycra, nylon - depending on where you want to use them.