Half life is not just used for radioactivity. Half life of a chemical reaction is the time it takes to reach 50% of its original concentration. That being said, in the environmental and toxicology space, they utilize that term to denote the timeframe where the chemical is of less concern to be in the atmosphere. Vinyl chloride’s volatilization takes 1 to 2 days in air. “If vinyl chloride is released to the atmosphere, it can be expected to exist mainly in the vapor-phase in the ambient atmosphere and to degrade rapidly in air by gas-phase reaction with photochemically produced hydroxyl radicals with an estimated half-life of 1.5 days.” Direct quote from this technical fact sheet: https://semspub.epa.gov/work/05/437069.pdf

Half life plays a role in medications we take too, for example, my mom takes Lisinopril (A.C.E. inhibitor to lower blood pressure) I took one by mistake one time, doc said not only does it get along with the few meds I take, it's half life is only 12.5hrs. Medications are are at their peak efficiency when most of their ingrediencies are working together to give yo the pest Their half life is considered when the ingrediencies are only working half potential. So bottom line if a medication has 6hr half life, it should affect you for appx 6 hrs, and take aapx 12 h for it to be out of your system.;