Cheaper than those piddly little cans of helmet mousse. Highly flammable! Illegal in many jurisdictions. Extremely effective.
Yes I know you can buy cans of denatured alcohol much cheaper but they have acetone and methylated ketones and who knows what else to make them poisonous, well more poisonous, and I’m not sure how those additives affect the helmet materials.
edit: turns out acetone is a solvent for polystyrene.