Exposure to contaminated combustion air will result in safety and performance-related problems. Typically you will run in to flame sense issues. The substances listed below will build on a flame sensor and insulate and stop it from sensing. Other issues will be certain acids that from from these substances can drop out in the heat exchanger and corrode it.  So if you are experiencing having to clean or replace flame sensors on a frequent interval, consider the space where the combustion air is coming from. Even if you are bringing in clean air on your intake, you may have to consider furnace box leakage.  

Do not install the furnace where the combustion air is exposed to the following substances:
 

• permanent wave solutions 
• chlorinated waxes or cleaners chlorine-based
• carbon tetrachloride 
• water softening chemicals 
• swimming pool chemicals 
• deicing salts or chemicals 
• halogen type refrigerants 
• printing inks
• cleaning solutions (such as perchloroethylene) paint removers
• varnishes 
• hydrochloric acid
• cements and glues 
• antistatic fabric softeners for clothes dryers 
• masonry acid washing materials