Possible Clues:

• Answer cry for help to protect finest building material
• Wally stores prime, oxygen-resistant material
• Chrysotile is the main natural form of this incombustible material
• A cry for help about worst poisonous substance
• Cancer-causing silicate once used for fireproofing
• Hazardous fibrous mineral
• Dangerous material as used by top mapmakers
• Hazardous material? Most desirable to enter a call for help
• A request for help about prime health hazard
• Fireproof material American bosses ordered? About time!