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Proponents of "no poo" claim that there is no medical reason for humans to wash their hair with synthetic shampoos, and that washing practices are determined by cultural norms and individual preferences, with some people washing daily, some fortnightly, and some not at all. From a clinical point of view, "the main purpose for a shampoo is to cleanse the scalp", though "most patients would disagree[,] stating that the purpose of shampoo is to beautify the hair".
The first synthetic shampoos were introduced in the 1930s, with daily shampooing becoming the norm in the US by the 1970s and 1980s. Proponents of "no poo" believe that shampoo removes the natural oils (sebum) produced by the scalp, causing the scalp to produce more oil to compensate. They also believe that regular shampooing causes a "vicious cycle" to develop as it becomes necessary to shampoo regularly to compensate for the excess oils produced by the scalp (which are produced in response to being stripped from the scalp by the previous shampooing). The time proponents believe it takes to break the cycle after adopting "no poo" practices varies, however a "two- to six-week period" is typical. These claims are unsubstantiated and not supported by current science and there is no mechanism in the scalp for detecting the amount of oil on the scalp and adjusting sebum production to compensate. 
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The purest form of "no poo" adoption is to use only water to wash hair, however there are other approaches possible by people wishing to avoid oil-stripping substances and chemicals that they consider unnecessary for the maintenance of their hair. Methods for washing hair without shampoo include washing with dissolved baking soda followed by an acidic rinse such as diluted vinegar. Also honey and various oils (such as coconut oil) can be used. Japanese traditional hair cleansing is with seaweed powder.
Following a 2007 radio interview that Australian Richard Glover held with Matthew Parris (a Times columnist "who hadn't shampooed for more than a decade"), Glover "decided to challenge his audience to go without shampoo for six weeks". Of the over 500 participants in the challenge, 86 percent reported that "their hair was either better or the same" following the challenge.
There are many variables that affect the texture and condition of hair. The frequency of shampoo use, the ingredients in the shampoo and their effectiveness, the use of styling products, the frequency of washing only with water; and if that is combined with occasional shampoo use; the temperature of the water, the mineral content of the water, the use of conditioner, the use of a hairdryer, climate, and also lifestyle and diet including vitamin and nutrient intake as well as intake of sugar ( [according to whom?]). This means that there is no clear cut method to achieving the desired results, and thus people should attempt to experiment with the various variables to find the most effective method for the hair type, color, style, and cut.
Cost is a reason some people decide to use "no poo" techniques instead of commercial hair care products.
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