The paperwork that comes with the Wave heaters says that dust and dirt can be the death of these heaters over time so you should ALWAYS keep it covered. The covers they sell fit snug and seem to work well.
Grease in the air (like, from cooking) and contaminants in the propane also are catalyst poisons. Not much can be done about the propane, since we can only assume that we're buying good propane.
The heater literature specifically mentions "alkaline" dust -- so dust-chemistry matters, apparently. I guess that means I have to be extra careful when camping on the Alvord or Black Rock playas -- alkaline dust galore.
I bought the Wave dust-cover at the same time I bought the heater. They make models that fit when used free-standing (as I do) as well as different models that fit for cabinet-mounts.