Dying your hair is not known to inhibit hair growth by itself, but dye can cause hair loss by damaging existing colour-treated hair. Hair underneath the scalp cannot be touched by hair dye, so it can’t cause hair loss by itself, but it may increase rates of shedding due to damage.
Hair loss and breakage after dying has three elements to it: first, the process of dying hair is usually intensive with a lot of pulling, combing and rubbing, and this process may be repeated every few months. This process can loosen hair and increase shedding when hair is in a certain growth phase (telogen). Secondly, hair dye contains ammonia and hydrogen peroxide, which can also loosen hairs in the telogen phase. Third, hair dye can weaken the shaft of a hair, which may increase breakage-related loss.
By far the most common cause of hair loss after dying hair is caused by disruption to the protein of the hair shaft. Any hair dye that lightens your hair from its natural colour contains large amounts of hydrogen peroxide, which is critical to remove the melanin pigments from hair, replacing them with the lightened colour. Anyone going from brunette to blonde will get shorter hair and need less hair cuts – hair will more frequently snap off at the ends, which are the oldest and easiest broken. This is known as weathering.
Hydrogen peroxide swells the cuticular scale and opens up hair shaft ports so that the chemicals can enter the hair shaft and they can undergo the necessary oxidation/reduction reaction to get lighter hair. The openings in the hair shaft close somewhat when pH neutralising conditioner is applied after the colour is applied, to reverse the hair shaft swelling (which is why home hair kits come with conditioner). The hair shaft, however, remains more porous than non-dyed hair, and porous hair sets the scene for further protein loss and weakening of the hair over time, especially with regular washing, heat-drying, curling, straightening and styling that hair undergoes.
Any hair that has been bleached very light from very dark will be weakened significantly at the scalp, which can cause hair loss temporarily until the hair regrows. If you are suffering hair loss due to dying your hair, you should stop dying your hair or pick a darker shade of colour.
If you are concerned about hair loss, and are seeing a dermatologist for advice, collect hair from four consecutive days in a bag to take to your appointment. Your dermatologist will examine the hair for those with an intact bulb – the little knobby bit you will find sometimes when you pull hair out at the root before it is ready. If less than 50 per cent of the hairs have a bulb, your hair is thinning due to breakage. If more than 50 per cent of your hair has bulbs, you’ll need to be assessed to find the cause.