In a recent study conducted by Good Morning America, skin care and make up samples picked up from ten US-based stores were tested by New York University's microbiology department. One out of every five samples - or 20 per cent - showed significant growth of mould, yeast, or faecal matter. The bad news is that the same bacteria might well be lurking in your own make up bag…
Not convinced? Here are just some of the nasties known to love nothing more than a warm, damp make up bag or an unloved lippie.
Pseudomonas aeruginosa is a bacteria commonly found in soil, water and warm, damp environments such as hot tubs and swimming pools. But if you think that steering clear of your local sports centre will help, think again: this bacteria is also found in old mascaras and can lead to painful eye infections.
Escherichia coli, or E. coli is normally associated with food, but it's also commonly found lurking in old make up - especially lipstick. When ingested, E. Coli can cause diarrhoea and abdominal cramps and, if left untreated, can eventually lead to organ damage such as kidney failure.
If you need convincing that sharing make up sponges or brushes is a bad idea, look no further than our next offender, staphylococcus epidermis, a naturally-occurring bacteria found on the skin of humans. This bacteria is especially easy to transmit through the shared use of lipsticks, eye liners and eye shadows and can lead to infections such as conjunctivitis and folliculitis - an infection of the hair follicles.
Staphylococcus aureus - otherwise known as MRSA - isn't just found on dirty hospital wards. This particular bacteria loves nothing more than an old, damp make up sponge - you're most at risk when using an infected brush or sponge on broken skin irritated skin.
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Use it or lose it
Holding onto products long after their use-by date doesn't just increase the risk of infection, but means that you're also using products which won't be performing as well as they should.
In 2010, a study by Debenhams found that while mascaras should be thrown out after four to six months, the average British woman keeps hers for a year.
Eye shadows, blushers, lipsticks and glosses shouldn't be kept for longer than 24 months, but on average, these products will be left to linger at the bottom of our make up bags for a shocking ten years.
It's often the products we use the least which we end up hanging onto.
"Lipsticks, eye shadow, blusher and similar powder products are the ones that most people keep beyond their use by date," agrees Chase Aston, the Body Shop's international make up artist.
"These products can usually last a long time, or if people are using limited edition shades that are only available for a short period of time, they tend to save them and only use them for special occasions, which can span over years, and the bacteria is built up over time."
Go through the contents of your make up bag at least once a month, and be ruthless - if you haven't used a product for more than a few weeks ask yourself if it's really worth holding onto.
While knowing the shelf life of products is important, remember that this varies according to the brand. "Liquid foundations in particular have a shelf life that varies between 12 and 24 months depending on the brand," points out Chase.
"You can check this by looking at the small symbol on the side of the packaging which is usually a small pot with a number and the letter M. But if the foundation has an odd smell, congeals on application or loses its texture and you have trouble blending, then bin it."
Be especially ruthless with any make up that's used on the eye area.
In a recent survey published in Optometry journal, researchers gave forty women different brands of mascara. The women were instructed to use the mascaras once a day. After three months, the mascaras were returned to the researchers and tested. Microbes were found in 33 percent of the mascaras, while staphylococcus or streptococcus were found in the majority.
Don't be fooled by talk of anti-bacterial agents either - while an increasing number of products do contain these, they only have a life span of around six months.
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It's not just the products we use on our skin that can transmit bacteria — it's the tools we apply them with, and the average germ loves nothing more than a warm damp brush.
"Make up brushes should be cleaned every three to five months," says Bobbi Brown. "To clean makeup brushes, use a mild household liquid soap. Use your hand as a cup for the warm, soapy water and swirl brushes in your palm until they're clean. Rinse well, then squeeze excess water from the brush. Air dry by laying the brush over the edge of a counter or table so that the bristles are open to the air on all sides."
Sponges can be washed in the sink, but ensure they're completely dry before going back in your make up bag. Consider investing in a multi pack of make up sponges and wash or replace your sponges at least once a month.
It's just as important to keep hair accessories clean. A recent study conducted in the UK found that there's more bacteria on the average hair brush than there is in a dog's bowl.
"By not keeping our brushes and combs clean, we're risking skin irritations, infections and the transferral of head lice," points out Mario Charalambous, technical director at the Richard Ward Hair & Metrospa.
But while most of us would never dream of going for more than a few days without washing our hair, we're not quite so vigilant with the tools we use to keep it under control.
"Don't allow hair to build up on the brush," advises Mario. "To remove, use a comb, or a brush cleaning tool. Soak the brush in an antibacterial liquid like barbicide, and wash with hot water. It sounds horrible, but once the brush is clean of visible hair, put it in your dishwasher on a high temperature. This will eradicate any remaining bacteria."
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Chase's top tips
Once you have used your lip-liner or eye-liner pencil (especially if you have shared), sharpen it. This removes the previous layer and reduces bacteria. Once a week take an alcoholic wipe and smooth over the tip.
Regularly give your make up bag a thorough wash to ensure that any spilled make up and harmful bacteria is washed away or destroyed.
For cotton/material bags wash in hot water using The Body Shop Tea Tree Facial Wash and for Plastic/Nylon types wipe thoroughly use cleansing wipes or an Alcohol/Anti Bacterial liquid solution.
Replacing and securing lids will ensure that there is never any make up spillage allowed to accumulate in the bag.
If your make up bag starts to smell, you could have mould or bacteria growing in the small, warm, dark corners. Clean it thoroughly and if the smell persists, invest in a new bag.
If you use a plastic or vinyl make-up bag, always remember to keep it out of the sun and away from warm/hot surfaces as bacteria will thrive in this type of warm and moist environment.