Latex is a natural biodegradable material. Once filled with helium, latex balloons only last the single day, if air filled they will lasts weeks (if not more) but will of course not float.
Foil balloons are made from Mylar, a synthetic material which is not biodegradable , once filled with helium they will last 3-5 days at their best and then will continue to float but with a more deflated/soft appearance.
Helium is non flammable and safe if used properly, helium must not be inhaled by children or adults no matter how much fun you think it can be. Helium cylinders are highly pressurised and heavy in weight. When being used they must be secured in an upright position to avoid falling over or tipping onto a person.