i have been useing my totobobo mask lately. it rocks. i use the densest filter and it a little hard to breath thru on really steep hills or exertions. kinda like your 5,000 feet higher than you are.
but the fact that you are breathing completely clean air makes it better. its one of the only masks that does that. it screens out every kind of pollutant and germs/bacterias.
you have to make sure you get the straps just right. one going up high and one low to keep it in place as your breathing gets going.
it was kinda cold the other day so i took a pic of the teprature gauge in my car.
so two hours of easy/medium effort not exceeding 170HR. the fenders did a great job of keeping me dry again as there are many puddles and snow melt on the street.