This Hedley & Bennett The Wake Up & Fight Mask ($45 for three) is a no-nonsense mask that doesn't take up much space (easy to put in your pocket on a walk, for instance). It has a slot to put in a filter, and it's washable (I've washed mine several times and it still holds up). Plus, Hedley & Bennett donates one for every mask purchased. Have multiples on hand when you travel.