Atmospheres containing acetaldehyde, ethylene, or flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode, having either maximum experimental safe gap (MESG) values greater than 0. 50 mm and less than or equal to 0. 90 mm or minimum igniting current ratio (MIC ratio) greater than 0. 45 and less than or equal to 0. 80.