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