Answered by: Maulana Mohammed Dilwar Hussain
Asalaamulykum. I want to know if keratin hair treatment is permissible. As some fatwa said it is not permissible because it coats the hair. Whereas some fatwa said it is okay to do and in-salon hair stylists said it doesn’t prevent water from reaching to hair. So what is the truth?
In the name of Allah, the Most Gracious, the Most Merciful
A keratin treatment sometimes called a Brazilian blowout or Brazilian keratin treatment is a chemical procedure usually done in a salon that can make hair look straighter for as long as 6 months. It adds an intense glossy shine to hair and can reduce frizz. The process involves getting your hair washed, then having a stylist brush the treatment onto wet hair where it will sit for about 30 minutes.
During a keratin treatment, your stylist will use a mixture of strong chemicals to apply a coat of protein (keratin) to your hair. This process seals your hair’s cuticle (protective outer layer), to prevent the movement of moisture in and out of your strands. Your stylist will rough-dry your hair, then finish it off using a flat iron, to seal and activate the coating.
Keratin treatments are often associated with formaldehyde. While they do not actually contain formaldehyde (because formaldehyde is a gas), many contain ingredients that release formaldehyde when they are heated or mixed with water. Examples include methylene glycol, methanediol, methanal and formalin.
These chemicals are extremely effective sealers — they are the ingredients that keep your hair smooth and frizz-free. However, formaldehyde is a known irritant and carcinogen, so we urge you to be cautious. There are some keratin treatments on the market that are formaldehyde-free, but they tend to be less powerful and long-lasting. 
If the hair of the head is not plaited, it is compulsory to wet all the hair up to the very base. If a single hair is left dry, Ghusl will not be valid. However, if the hair of a woman is plaited, she is excused from loosening her plaited hair. It would still be compulsory for her to wet the base of each and every hair. If she fails to do so then the Ghusl will not be valid. [Al Mabsoot; Volume 1, page 24] 
From what has been understood by the above information in regards to the procedure in which the keratin treatments are carried out, it would not be permissible as the chemicals used effectively seal the follicles of every hair preventing water from penetrating into them.
In the scenario above, plaited hair was permitted as long as the base of every hair was accessible enough so it could be washed. However, in the case of keratin treatments, every strand of hair becomes sealed with the chemicals so water is no longer able to penetrate them. Thus, wudhu and ghusl would not be possible. Alternative treatments are available which have fewer side effects. That would be the advice to take.
Only Allah knows best
Written by Maulana Mohammed Dilwar Hussain
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 قلت أَرَأَيْت إِن اغْتَسَلت الْمَرْأَة وَلم تنقض شعر رَأسهَا إِلَّا أَن المَاء يبلغ الشّعْر قَالَ يجزيها