Acetylene
Chemically pure acetylene is a colorless gas with a faint ether-like odor. Technical acetylene has a sharp specific odor due to the presence of impurities, in particular phosphine.
1.7.1. Acetylene is a weak narcotic. Inhaling air containing up to 5% acetylene does not cause any painful sensations. At high concentrations, acetylene acts as a suffocating gas.
1.7.2. Acetylene explodes when mixed with air and oxygen at acetylene concentrations within the range of 2.2 - 100% by volume
Source: "Instruction on labor safety when handling oxygen and acetylene cylinders" (approved by the Russian Ministry of Labor 14.05.2004)