The problem with PAO guidelines (and even expiration dates) is that though they're a good start, they don't account for how you store and use your cosmetics — both of which can affect shelf life. The Food & Drug Administration warns consumers that cosmetic expiration dates are simply "rules of thumb," and that a product's safety may "expire long before the expiration date if the product has not been properly stored."
Dr. Zein Obagi, a Beverly Hills dermatologist who's been in practice for more than 30 years and created Obagi skin-care products, warns against supersizing. "Women want to buy something that they can use forever so that they don't have to buy it again," Obagi says. "Unfortunately, that doesn't happen. That's why they put cosmetic products in small containers because they should be used in a short period of time." So sidestep the industrial-sized face cream — better instead to buy the small size more often.
Obagi says that if you keep any product too long or expose it to air or direct sunlight it will degrade. "Chemical changes occur in the formula, and there's going to be evaporation and a high concentration of chemicals especially irritating for sensitive skin and in sensitive areas like around the eyes," he says. "Vitamin C, retinol, hydroquinone, glycolic acid — all ingredients are vulnerable."
Obagi says that vitamin C becomes unstable and loses efficacy after a few weeks.
"Aside from a dry mineral powder, everything is susceptible to being highly contaminated," he says, adding that one solution is airtight packaging. Air can cause contamination, as can dipping your fingers into containers.
If you keep cosmetics in warm, humid or hot areas — for instance, your bathroom, like just about everyone does — you're risking problems. "You may open the product for just one minute and that humidity will be a fertile ground to allow mold, bacteria, fungus and streptococcus to grow," Obagi says. "You find these contaminants in old makeup and skin-care products." Using products like these makes you vulnerable to infections, acne, pustules and scarring. "Even if a product is kept away from sun and heat I wouldn't use it after two years even if it was sealed," Obagi says.
And don't forget your loofah, sponges and washcloths as you're cleaning out your beauty products. Edward J. Bottone, professor emeritus of medicine/infectious diseases at Mount Sinai School of Medicine, published an article about infected loofahs in the Journal of Clinical Microbiology. "Loofahs are especially vulnerable to bacterial growth," Bottone says. "These natural sponges have many nooks and crannies that, especially when moist, invite bacteria."
Bacteria feed off carbohydrates and proteins in the loofah — including dead skin. "I once had a woman come to me who had a horrible rash and infection and couldn't figure out what it was from," Bottone says. "I had her bring in her loofah and other wash items. Her loofah was green with mold and bacteria. If you have little cuts in your skin and wash with an infected loofah you can be infected.