Semi-permanent hair dyes are a great hair colouring alternative because they require no ammonia or peroxides, which means they are way more mild on your hair and scalp when compared with permanent hair dyes!
It’s still hair dye right, so how can it been “better” for your hair? Well this is where we are able to differentiate the molecule structures of semi-permanent vs permanent hair dyes. There is no need to get overwhelmed by the science behind it all, because it’s actually not too complex to wrap your head around!
Semi-permanent hair dyes are made up of both small and large colour granules that penetrate the cuticle and lay on the outer edge of the cortex. Permanent dyes are made up of purely small colour granules which, when combined with hydrogen peroxide, swell and join together to then become trapped deep within the cortex of each hair follicle. Permanent hair dyes do not wash out, however, as your hair starts to grow out from the roots, so will the dye. So to put it simply, if the colour molecules are small and mixed with hydrogen peroxide they are going to effectively penetrate the hair follicles and are not going to be washed out.
The essential difference between semi and permanent hair dyes is the presence of developer. Permanent hair dyes contain both colour and developer, whereas semi-permanent dyes do not contain the developer, and therefore do not have the ability to open the hair shaft in order for the colour pigments to enter the hair follicle. Semi-permanent dyes are not dangerous and will not lift the natural colour pigments of your hair, it will simply deposit colour onto the hair.
There shouldn’t be a significant amount of concern towards semi-permanent hair dyes, as it is highly unlikely that there will be any long-term impacts. However, it is important to note that too much of anything can be damaging, so you should not be applying semi-permanent hair dyes more than once a month and you should always do a patch test first! Find more information on patch testing here.