How to prevent acne when I spend most time in a filthy environment and what is the best natural cleanser?

Filth doesn't = acne. Acne occurs when skin glands produce more oil, but the openings of the glands are blocked by little plugs of dead skin cells (comedones). Skin oils leak sideways into the skin, causing inflammation that brings in lots of white blood cells, forming a pustule. Dirt (filth) doesn't cause acne but some chemicals, hormone changes, emotional stress, some medications, or excess skin rubbing can cause it.
Cetaphil. Cetaphil is a non-soap cleanser that can be used often without drying or otherwise harming the skin. You can use it several times a day if needed. You don't even have to use water if it's not available; just gently wipe it off. Good luck.