Nail polish may look pretty, but it contains various toxic chemicals and is classified by the EPA as hazardous waste. Never throw it in a garbage can or pour it down the sink, as it may pollute the local water supply. Instead, take old nail polish, along with unwanted nail polish remover, to a household hazardous waste facility for safe disposal that won't harm the planet.
Another Option
If you don't live near a hazardous waste disposal plant, you can still dispose of nail polish safely, according to the website Unclutterer. Uncap the bottle and let it sit until it dries out completely, becoming a solid. Next, screw the cap on tightly and throw it away; since it's no longer a liquid, it won't leak and contaminate water or soil.