“Why not roll some canisters of CN-20 down there,” suggests Vasquez after the first disastrous journey into the Alien hive, “nerve gas the whole nest?” Ripley is not so certain. “No good,” she says, “How do we know it’ll affect their biochemistry?” The survivors proceed to dismiss the idea of gassing the hive, and opt to nuke the site from orbit. “It’s the only way to be sure.”
As pointed out by AVPG user, SiL, Ripley in fact gassed one Alien already, during Alien’s climatic scenes aboard the Narcissus. Most folk -myself included- assumed this to be hot, pressurised air. But looking at the buttons which Ripley pushes to gas the Alien reveal that some nasty chemicals are involved in the mixture. Initially, we assumed these were pesticides of some sort, which would have been an apt gag, considering that the Alien was inspired by ants, wasps, and other insects, (even if said gag didn’t make much sense in-universe). However, a closer look reveals that three of the compounds (Iodine, Methyl chloride, and Nitrosyl chloride) are far deadlier to the Alien (and Ripley) than any pesticide.
According to Wiki, Iodine “is an oxidizing irritant and direct contact with skin can cause lesions … Solutions with high elemental iodine concentration such as tincture of iodine and Lugol’s solution are capable of causing tissue damage if their use for cleaning and antiseptics is prolonged.”
As for Methyl chloride, “brief exposures to high levels of methyl chloride can have serious effects on the nervous system, including convulsions, and coma. Other effects include dizziness, blurred or double vision, fatigue, personality changes, confusion, tremors, uncoordinated movements, slurred speech, nausea, and vomiting.”
Nitrosyl chloride is “Very toxic by inhalation … Dissolves into and reacts with moisture in the air or with water to form hydrochloric acid … Very toxic gases are generated when heated … Inhalation causes severe irritation of respiratory tract and damage to mucous membranes,” (Ripley may also want to take note: “As an immediate precautionary measure, isolate spill or leak area for at least 100 meters -330 feet- in all directions.” Oops.)
All in all, and considering the Alien’s pained, screeching reaction to exposure to the vented gasses aboard the Narcissus, maybe a few canisters of CN-20 wouldn’t have been such a bad idea after all.