How to know if your hair products have expired.

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So you’ve worked out that it’s probably high time that free mascara you got with a magazine four years ago went in the bin, but have you ever thought about the lifespan of your hair products?

While there’s plenty of information around about the expiration of makeup, when it comes to hair it gets a little more confusing — with often less-clear varying dates dependent on product type, texture, use and storage. Eek.

So should you be immediately chucking out all your hair products to start from scratch?

Unless they’ve been collecting dust on your bathroom shelf since ’06, probably not. Here’s what you need to know.

How do I work out the expiration date?

First things first, flip the bottle over to the ingredients.

“On a lot of packaging these days, there will be a little sign on the back of the bottle stating the amount of months until it expires,” sayas Tori Cini at Edwards and Co in Alexandria.

Usually this is in the same style as you’d find on your makeup products – a number contained in a little jar.

What if there's no number?

Unfortunately, not all products have the handy jar number to guide you — but there are general rules you can stick to.

While regular use means it's probably rare for your favourite shampoo to last longer than a couple of months, beware the more sporadic lurkers on your shower floor.

"Shampoos and conditioner are more likely to last around 12 months," says Cini.

"But aerosols can last longer because they are sealed, lasting nearly two years after being opened, whereas some pastes will only last six months."

If a product remains sealed, it's good to hang in your cupboard for up to three years — but the minute it's opened it drops to around 12-18 months.

Watch: Mia Freedman's hairdresser explains the secret to a good haircut. (Post continues after video.)

What signs should I look out for?

If you're concerned, have a closer look at the product before you lather it all over your hair.

"Smell is usually the biggest indicator that a product has gone off. Discolouration is an indicator too, and it typically effects the consistency," says Annie Shortall, Creative Director of m a k e.

"It’s also always smart to keep spray nozzles and pumps clean to prevent any bacteria contamination that would cause a product to go off before its expiry date." Noted.

Obviously if you see any signs of mould or fungi, discard. Immediately.

"Sometimes this is visible, other times it can not be seen. When this happens, you can expect products to have a rancid smell - they may smell acidic and in extreme cases even begin to smell like rotting meat," says Al'chemy's Botanical Chemist Annabelle Personeni. (Post continues after gallery.)

This can often be a by-product of the resourceful act of adding water to use that little bit of product left at the bottom of the bottle.

"Doing this when there is only a small amount of product left can accelerate the likeliness of bacteria to grow. By diluting the product, you are diluting the preservatives within the formula making the product more susceptive to bacteria and more likely to develop mould," she says.

These changes can happen as quickly as two days after adding the water.

How does storage affect my products?

If you really want them within easy access in the shower (and let's face it, we all do) start by picking up your products from the shower floor.

"Always keep products away from direct sunlight and moisture as that’s where bacterias can build up and thrive. So in the shower try to keep products on the shelves and not on the floor. On the floor can mean water can potentially get into the packaging easier and create bacteria growth," says Shortall.

Temperature can also play havoc with both the expiration and efficacy of your products.

"Humidity and heat as well as direct sunlight can affect products if they're left for an extended period of time such as more than six months," says Personeni.

"Sunlight and heat will degrade fragrance and damage the preservatives - meaning products are more open to bacterial contamination."