QotW: Do you really need to wash, rinse, and repeat?

20 July 2020

SHAMPOO

a hairdresser washing someone's hair with shampoo

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Question

I'm wondering if there's any science around the wash, rinse, repeat method that manufacturers tell us is the best way to use their shampoos. Or is it just a really clever way of getting us to use more? Thank you!

Answer

Phil Sansom's been getting down and dirty to answer Julie's question...

Sally-Ann - There is actually science behind the double hair wash method, and it is not a ploy to get you to buy more shampoo.

Phil - That is Sally-Ann Tarver, who is a trichologist - someone who studies and treats the hair and scalp. And while Sally-Ann says this ‘wash, rinse, repeat’ isn’t just marketing hocus-pocus, it’s also not always necessary.

Sally-Ann - It is really dependent on how often you wash your hair. If it’s short and you wash it every day, then no, you don’t need to shampoo twice, as your hair hasn’t had time to get oily. If you use a lot of product, you haven’t washed it for a couple of days, or it’s long or thick, you will need to wash it twice.

Phil - Now the reason has to do with how the soap or cleanser in the shampoo works.

Sally-Ann - The soap or cleanser molecules, micelles, can only hold so much dirt. Micelles are molecules that have one end electrostatically drawn to dirt and the other end drawn to water. As you shampoo your hair the micelles are attracted to the dirt / oil / product and kind of engulf it. Then when it’s rinsed out, the water loving micelle end is drawn to the water and the dirt is rinsed away with it.

Phil - The micelles can only hold so much dirt at once - so to really get at that stubborn oiliness, you might be better off doing two washes, just with less shampoo.

Sally-Ann - The first shampoo you may not get a lather but those micelles are attracting a lot of dirt which is then rinsed out. The second shampoo removes any remaining oil, product, etc.

Phil - Eva Proudman from the Institute of Trichologists agrees - and she says the advice changes depending on what you get up to.

Eva - If you love hair products and lavish them on your hair then two shampoos will most probably be required to ensure that they are fully removed. If you wear a hat or head covering all day, or work in an environment that produces pollution, then again, the 2-stage shampoo is for you. Those who suffer with dandruff, seborrhoeic dermatitis or psoriasis - regular shampooing can help to manage the scalp environment, so it’s a must. If you are  experiencing a flare up in symptoms, wash, rinse, and repeat can really help to calm the flaking, itching, and inflammation.

Phil - Thanks Eva Proudman and Sally-Ann Tarver. Next time we’re digging through the dump - to answer this question from Johnny:

Johnny - Does burying paper in a landfill sequester carbon?

Eva - Can you help out with Johnny’s question? you can ask via our website by going to nakedscientists.com/question, email chris@thenakedscientists dot com, find us on Facebook, tweet @nakedscientists or join in the debate on the forum - the naked scientists dot com slash forum.

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