We’ve all heard hair dye horror stories of breakage, dryness, and all-around signs of damage.

But if hair dye is so bad for you, why can it make your hair look so good? Here’s what you should know about the connections between hair dye and hair health.

Proteins make up about 95 percent of a dry hair strand. These proteins act like a roof’s shingles, protecting the strand underneath from:

  • moisture
  • heat
  • UV rays from the sun

Applying permanent or demipermanent hair dye or bleach creates a chemical reaction that causes these protective proteins to lift, allowing chemicals to penetrate the hair strand. There, the hair dye alters the chemical makeup of the hair. The result is a color change.

Permanent or demipermanent hair dye is a different process from temporary or semipermanent hair dyes. Temporary dyes leave color on your hair but don’t usually penetrate deeper layers (though they can still damage your hair). This is true for alternatives to traditional hair dye like Kool-Aid, carrot juice, and even coffee.

Altering the hair’s protein structure through permanent and demipermanent dyeing can cause side effects that include:

  • loss of hair strength
  • less ability for your hair to handle heat styling
  • reduced hair thickness
  • increased roughness of hair follicle

Keeping this in mind, the hair dyeing process changes the hair by nature. The short- and long-term effects of this depend on your hair structure.

If your hair is already thin or brittle, dyeing can make it even weaker. Thick hair can also experience side effects from dyeing because the dye may take longer to “lift” the hair proteins, and it may require longer applications.

The following are some chemicals commonly used in the hair dyeing process.

Bleach

Bleaching hair alters the pigments that give hair its color. A 2020 study found that bleaching could affect:

  • hair strength
  • moisture
  • hair’s ability to withstand heat

The degree to which bleach damages the hair depends on the pH (or acidity) of your hair and can also be affected by the pH of the environment around you.

Bleaching is usually done by applying hair dye that contains hydrogen peroxide. Hydrogen peroxide is an oxidizing agent. This means it activates the hair color, but the oxidizing process does cause damage.

Henna

Henna is a natural semipermanent hair dye. However, “natural” doesn’t mean less damage.

A 2019 study found that henna dyes can damage your hair, resulting in coarser, more brittle hair. When the henna dye was left on longer than an hour, the damage was similar to that of hair bleaching.

Henna can also cause contact dermatitis and skin irritation, especially if you’re allergic to any of the compounds or ingredients in henna ink.

P-Phenylenediamine

P-phenylenediamine (PPD) is a chemical compound commonly found in hair dyes that require an oxidizer like hydrogen peroxide.

Some people report greater incidences of allergic reactions due to PPD hair dyes.

When it comes to lightening your hair, the more you have to bleach it, the greater the damage.

A 2019 study that used microscope technology to look at degrees of bleaching found that hair fibers were most damaged by prolonged bleaching, especially the kind needed to change very dark hair to lighter hair. This is why many hair coloring specialists do not recommend significantly lightening the hair at one time.

Going from lighter to darker isn’t necessarily less damaging because you are still changing the hair’s structure. It’s important to take care of hair that’s been dyed darker as well.

History of prior hair damage from heat styling or harsh dyeing practices can increase the likelihood of further hair damage.

Here are some ways you can help protect your hair:

  • Use hair dyes with conditioning agents. Conditioners in hair dyes help minimize the degree of hair damage. An example is hydrolyzed silk proteins.
  • Limit heat styling. Heat styling can further weaken hair. Limit your use of heat styling tools, such as:
    • hair dryers
    • curling irons
    • flat irons
  • Apply thermal heat protectant sprays or lotions. Apply these to the hair before heat styling.
  • Allow time to pass between hair dyeing appointments. Less frequent hair dyeing appointments can help minimize damage.
  • Use moisturizing shampoos and conditioners. These products help the hair hold moisture after dyeing. Highly fragranced shampoos with high amounts of detergents (they usually have a lot of lather) tend to be more damaging.
  • Choose color-specific shampoos, conditioners, and other hair care products. These are meant to help protect color-treated hair.
  • Consider dyeing your hair closer to its natural shade. This may help limit damage whenever possible.

In rare instances, hair dye can cause severe allergic reactions.

Swelling of the lips and eyes are early indicators of allergic reactions. Seek emergency medical attention if you start to experience problems breathing after applying hair dye.

It’s possible to perform a “patch test” of hair dye by applying it to a small area before dyeing all the hair. Keep in mind that not all people who have an allergic reaction to hair dye have a reaction from a patch test.

Hair dyes can cause hair damage. Practicing hair-friendly techniques like limiting heat styling and conditioning the hair can help minimize damage whenever possible. Dyeing your hair closer to its natural color may also limit time exposure to hair dyes and help minimize damage.

If you’re concerned about your hair’s health, talk with a stylist about dyes with conditioners or other methods to keep your hair looking and feeling healthy.