Even though it may seem easy to just toss that old bottle of fingernail polish in the trash, this is not something you should do. Nail polish contains toxic chemicals including dibutyl phthalates, formaldehyde, and tuolene that, if not disposed of properly, can leach into the ground and pollute the soil and groundwater. Never pour nail polish out or throw the bottle into the trash.
To dispose of old fingernail polish, the bottles need to be brought to a hazardous waste collection facility. Search online for one of these facilities in your area. Some communities or cities have hazardous waste collection days to collect old cleaning supplies and similar items, and this is another opportunity to properly dispose of your old fingernail polish. If this is impossible for you, some discussion forums suggest leaving the cap off the nail polish to allow it to solidify, then throwing away the bottle. This should not be your first choice, though.
You may not need to throw your old fingernail polish away at all. If the nail polish has started to look dried out and separated, a few drops of nail polish thinner mixed into the bottle can make the fingernail polish appear new again. Do not make the mistake of mixing nail polish remover in the bottle; the nail polish remover will not mix with the polish and the color will be ruined. Nail polish thinner can be found in the cosmetics department of most stores, or can be purchased at a salon. Thinning your nail polish slightly can also make the polish stronger on your nails and less likely to peel off.
If your old nail polishes are still fine, and you've just grown tired of the color or the brand, you might consider giving them to a teenager. Teens enjoy painting bright colors on their nails, and they will probably get some good use out of your old nail polishes and you won't need to throw them in the trash. You might also use old clear nail polish to stop runs in stockings or at the end of shoelaces to prevent fraying.
If you are concerned about the health effects of some of the chemicals found in nail polish, search online for more eco-friendly brands. Some popular nail polish brands are removing or cutting back on the toxic chemicals in the polish, and these results can be found on cosmetic databases online. These databases list the ingredients found in cosmetics and gives them a numerical rating. Even if you do purchase a more eco-friendly fingernail polish, you still need to properly dispose of the remaining polish at a hazardous waste collection facility.