This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Toxicology - 1, Subject name: Forensic Medicine
All are chemical Asphyxiants, except? 
Select and explain. 
A. Helium, B. Carbon Monoxide, C. Cyanide, D. Carbon disulphide
A: Helium
Answer: A: Helium 
Explanation: Asphyxiants (1) IRRITANTS: The gases injure the air passages or lungs or both, and produce inflammatory changes.smoke, tear gases, ammonia, formaldehyde, chlorine, phosgene, nitrogen dioxide, sulphur dioxide. (2) CHEMICAL ASPHYXIANTS :The examples are carbon monoxide, hydrogen sulphide, arsine, carbon disulphide, and cyanide. (3) SIMPLE ASPHYXIANTS : They are carbon dioxide, methane, helium, nitrogen and nitrous oxide. Symptoms appear when these gases exceed 20 to 30% of inspired air. (4) VOLATILE DRUGS: absorbed through blood and toxic to internal organs like liver, kidney and CNS. They are aliphatic hydrocarbons, halogenated hydrocarbons and aromatic hydrocarbons. (5) SYSTEMIC POISONS : These are insecticides, arsine, stibine.