You can't get decent performance from a respirator type device without a proper seal. The negative pressure created by inhaling is too large a volume and a tiny

You can't get decent performance from a respirator type device without a proper seal. The negative pressure created by inhaling is too large a volume and a tiny little fan won't be enough to push that much air fast enough, especially through any kind of filter. If you look at forced air respirators their external units have to be pretty bulky and that's with a full face mask. If you want clean air I'm afraid you'll forever have to walk around like an extra on Contagion.