I think the others are right, unless you spend the big bucks for good quality filters, it is best not to go the compressed air route.
I'm sure I will get some flak, but I built my own fresh air system and have been using it for years. I bought a new vacuum cleaner motor on ebay for about $70, the impeller is sealed from the motor so you are not breathing air that passed through the motor. I built a wooden box with a switch, an inlet air filter, discharge pipe and a bypass valve. I use the cheap corrugated medical blue ventilator hose connected to my surplus fire department Scott full face mask. All told, I guess I have about $150 in it.
It is noisy, but the air is cool and clean. Still the easiest way is to simply buy a Hobbyair. It seems like a lot of money but the peace of mind is great not to mention your body will thank you too.