All traditional shemaghs are made of cotton. It is just plain comfortable, practical in most conditions, just enough. Some of the cheaper ones I found in Iraq were too open weave to be much more than a sunshade and mosquito net. However, the ones marketed by brigade quartermasters and other such companies are more substantial, enough so that they will do an effective job as a sand block. You will get some blow-by, but too tight a weave, like a linen, get's plugged up and difficult to suck air through pretty quick.

Man, air travel is getting scary...