The air filter is located on the back of the lower build chamber. A fan inside the printer pulls air in to cool off the printer components. This filter keeps contaminants out of the air being pulled in. If the filter gets resin on it, it must be replaced IMMEDIATELY.
CAUTION: Failure to change the filter when prompted could lead to a clogged filter, which would block the cooling fan from taking in air. This could cause printer parts to overheat and fail. Similarly, installing a damaged or used air filter may result in overhearing or debris getting in the lower print chamber.