The reason why gas masks require training is that airflow inside the mask is perceived as an impediment by our instincts, so that we tend to reflexively take the mask off when we should not.
Less dramatically, a friend is dealing with a severe pollen storm. She can use a painter's mask or the like, but it becomes fatiguing to breath through it.
This simple build is intended to alleviate that problem by providing positive pressure before the mask's filter.
So let's get a mask.
First, build a battery. You can use either a 5V power bank or do what I do here, use an old 7.2V NiMH battery and build a USB charger for it with a step-up regulator that I had sitting around.
Fourth, add a plug to the battery and fan. Since this fan has some spin-up time to it, we're just going to leave it running - we have a beefy battery after all. This makes for a simpler build and increases perceived ease of breathing in, which is our primary goal.
Fifth, put them together. If this is intended to help with allergies, you may want to remove the carbon from the filter and add one or two layers of gauze instead.
Sixth, ship it. It barely fit in a USPS flat rate box.