Fumigant Chemistry, Toxicology, and Label Comprehension
Physical behavior drives placement
A fumigant's movement is predicted largely by molecular weight relative to air (about 29). Methyl bromide (MW ≈ 95), sulfuryl fluoride (MW ≈ 102), and chloropicrin (MW ≈ 165) are heavier than air and sink into crawlspaces, pits, and low corners; hydrogen cyanide (MW ≈ 27) and phosphine (MW ≈ 34) are close to neutral and mix more evenly. Vapor pressure and boiling point determine whether a product is released as a gas or flashed from liquid, and whether a vaporizer is needed. Sorption to commodities and building materials removes gas from the air and can leave the top of a structure or center of a commodity stack underdosed.
Toxicology you are expected to know
Methyl bromide is an alkylating agent: delayed pulmonary edema and neurologic effects, no reliable odor. Sulfuryl fluoride is odorless and acts via fluoride; it is a central nervous system toxicant. Phosphine, generated from metal phosphides by moisture, inhibits cytochrome oxidase and can cause hemolysis and pulmonary edema; it also corrodes copper and reacts to produce ammonia. Chloropicrin is a lachrymator used as a warning agent, not a stand-alone structural fumigant.
Label comprehension
Read the label as a checklist: active ingredient and concentration, signal word, PPE table, rate range, temperature limits, required exposure, clearance level and who may measure it, storage, and disposal. Traps include assuming a warning agent makes a gas safe, treating methyl bromide as a cholinesterase inhibitor, and using a rate without the exposure period that rate assumes.
Rule of thumb: MW greater than 29 means monitor low; MW near 29 means monitor everywhere.