Best heat protectant: keep the heat off your color

Does a heat protectant actually do anything?

It does, and it helps to know what. A protectant does not put a shield between the iron and your hair. What is measured is a result, not a mechanism: hair pre-treated with certain polymers breaks less after ironing.

And it helps to know who measured it. That work is signed by a polymer manufacturer, not an independent lab: breakage drops noticeably. Drops, not disappears. No brand on this list publishes a study like that for the bottle it sells, and the one that exists is not impartial either.

And one thing matters more than which brand you pick: damage starts sooner the higher the temperature goes. In a study using a curling iron, turning the heat up barely improved curl hold, but the onset of damage was strongly temperature dependent. Translated: before you change bottles, turn your iron down. With one warning, because the gain is easy to lose: do not make up for it with more passes. And if your hair comes from bleaching or highlights, that temperature is set by the person who colors it, looking at your hair, not by a label.

What the asterisk does not back

Less than it looks. Seven of these eight brands print the same ceiling, and not one says what hair it tested, how many passes it survived, or what counted as protecting.

That figure is the ceiling of an ordinary flat iron. It is a compatibility label, the “works up to here” kind, not the result of a measurement.

The asterisk is what gives it away. When there is one near the number, it almost never backs that number:

• On Kérastase, the asterisk belongs to the “96% smoother”, compared with untreated hair.
• On Redken, to the “13x smoother”, versus a non-conditioning shampoo.
• On ghd for coloured hair, to the protection from UV rays.

Only one brand says even what kind of test it ran, and it stops at calling it instrumental. None states the conditions.

So do not shop on temperature: it is the same on almost all of them. Shop on what each one does declare, which is what this page puts in front of you.

Redken and L’Oréal: same group, much of the list shared, a different formula

This is what came out of putting all eight ingredient lists in one table.

Redken’s Acidic Color Gloss protectant and the US formula of L’Oréal’s Heat Slayer share 23 of the 29 ingredients the retailer Ulta publishes for the first. The first four run in the same order. Among those 23 are xylose, arginine, serine and glutamic acid. L’Oréal’s European formula is a different one and carries none of those four: it is called Dream Long in Spain and also Heat Slayer in the UK.

Both brands belong to the same house: Redken sits in L’Oréal’s professional products division, same as Kérastase.

Now, what that does NOT mean, and it matters:

• They are not the same formula. An ingredient list runs in order of concentration only above one percent. It never says how much of anything is in there.
• It says nothing about pH, which is exactly what Redken highlights: the word acidic is in its name.
• The list of 29 is the one the retailer Ulta publishes, and it matches the one on Redken’s UK site.

What you can do with the finding: pay the difference knowing what it buys you.

Keep the dryer far and keep it moving: that is the part of heat protection that costs nothing.

Every page asks for something different

Its own page tells you, and they do not all say the same. Some ask for damp hair, some for towel-dried, and one takes it either damp or dry.

But there is a finding from outside the brands that helps you decide. At the University of Manchester they ironed hair treated with a water-based spray and with an alcohol-based one. Chemical damage dropped the same with both. Structural damage was worse when the hair was wet, and the likely cause is water flashing off all at once.

It does not let anyone say one product protects more than another: they tested two laboratory sprays, not bottles off a shelf. What does carry over applies to any of them: there has to be a drying step between the product and the iron. The protectant goes on where its page says, usually damp or towel-dried. Then the hair is dried all the way. And only then does the iron come in.

And watch out, because there is a contradiction between markets here. The American page of Heat Slayer says to apply to damp hair and does not say to dry it before flat ironing. The European page of its sibling Dream Long says dry or slightly damp. Follow the page of the bottle you are actually holding, and keep the drying step in between.

What is actually measured about dye

This is where precision matters, because it is the question that brings most people here.

What is measured is that heat changes the color of the hair itself:

• A plate at 172 °C darkened virgin hair in two minutes.
• A thermal treatment at 81 °C yellowed grey hair, on both types tested.
• Across repeated washing and drying, the color of undyed hair shifted mostly towards lighter.

What we did not find is a study measuring that irons or dryers fade oxidative dye. We looked on PubMed and did not find it. So we do not claim it, however often it gets repeated.

One detail from the same work that surprises: blow drying at a distance and on the move did less damage than letting hair air dry. The dryer is not the problem; the dryer held close and still is.

What this page is not

We have not tested these products. We have no lab and no test panel, and faking a test we did not run would be worse than saying so.

What we did do is read the eight official pages and put them in one table. Reading one label is no achievement; reading eight at once shows what you miss on a single one: that the figure nearly all of them print is the same one, and that the Redken list and the US Heat Slayer list start identically.

And it is worth saying what a bottle does not do. A heat protectant does not replace your colorist’s judgement on what your hair can take. If you are coming off a recent lightening service, that conversation is worth more than any of these eight.