Butyl Methoxydibenzoylmethane

Butyl methoxydibenzoylmethane is a synthetic sunscreen ingredient that provides sun protection in the UVA range. This type of radiation is present all day long, and penetrates deeply into skin, where it causes a cascade of damage that leads to many signs of ageing.

Better known as avobenzone, butyl methoxydibenzoylmethane is globally approved. In fact, it’s one of the most widely used UVA filters in the world and has been rigorously tested for safety and efficacy, with further safety studies ongoing.

In the United States, it may be used as an over-the-counter sunscreen active up to 3%. Its maximum permitted concentration extends to 5% within the European Union, where it’s been in use since 1978 (it was FDA-approved in 1996).

Many studies have shown that avobenzone becomes unstable when exposed to UV light, one reason reapplication during long periods of time outdoors is necessary. To increase its photostability, UV absorbers such as octocrylene and non-UV filters (e.g., diethylhexyl 2, 6-napththalate) often accompany avobenzone in a sunscreen formula.

Synthetic and natural antioxidants (such as vitamin E ubiquinone, glutathione, and diethylhexyl syringylidenemalonate) have can also improve avobenzone’s photostability, as can the proper choice of emollients, one example being caprylic/capric triglyceride. Other ingredients used to stabilize avobenzone and allow it to be used with other filters such as octinoxate include polyester-8 and ethylhexyl methoxycrylene.

Avobenzone is one of several sunscreen ingredients currently undergoing further safety testing under the purview of the United States Food and Drug Administration (FDA). This testing is to gain a better understanding of the systemic absorption, metabolism, and elimination of these sunscreen actives when small amounts enter the body via topical use. It’s important to know that the presence of this or other sunscreen actives in the body does not mean your health is at risk. It is anticipated that the additional testing being done will reaffirm the safety of these ingredients; however, those who remain concerned can choose sunscreens with mineral actives (titanium dioxide and zinc oxide) which are not included in the FDA’s new call for additional testing.

Note: this UV filter is also known as Parsol 1789, but avobenzone is the official name when used in sunscreens.

Avobenzone works by absorption, not by reflection. The molecule can sit in two interchangeable shapes: a chelated enol form and a keto form. In a well-made sunscreen it exists overwhelmingly as the enol, and it is this shape that captures UVA photons. When a photon lands, its energy is taken up by the molecule's electrons; the molecule then sheds that energy harmlessly — largely as an imperceptible amount of heat — and drops back to its resting state, ready to absorb again. Less UV energy therefore reaches the skin beneath the film.

The complication is that not every excited molecule returns cleanly. A share flips to the keto form, which absorbs UVA poorly, and some fragment altogether. Studies have shown that avobenzone's UV absorbance falls when it is exposed to UV light. That is what photo-instability means in practice, and it is why reapplication is emphasised so heavily.

Formulators design around this rather than accept it. Photostabilisers such as octocrylene, ethylhexyl methoxycrylene, diethylhexyl 2,6-naphthalate and triazine filters soak up or drain off the excess energy before the molecule can break down, while antioxidants including vitamin E and well-chosen emollients such as caprylic/capric triglyceride help keep it in its active enol form. A properly stabilised modern formula behaves very differently from raw avobenzone in a dish.

Butyl methoxydibenzoylmethane only ever reaches you inside a finished sunscreen, so the practical questions are really about how you use that sunscreen.

• Use enough. Roughly half a teaspoon for face and neck, or the two-finger rule — a strip of product along the length of your index and middle fingers. Applied thinly, the UVA protection printed on the label is not the protection you actually get.
• Apply it last in the morning. After serums and moisturiser, before makeup, and give it a couple of minutes to set.
• Reapply roughly every two hours in strong sun, and after swimming, sweating or towelling. Because the molecule slowly degrades in UV light, reapplying matters more here than with mineral filters.
• Check it is paired properly. It gives little UVB cover alone, so look for UVB filters further down the list and a UVA-in-a-circle symbol or a high UVA star rating on the pack.
• Mind your whites. Residue can react with iron in water and leave rust-coloured marks on fabric, so wash flannels and light-coloured shirts promptly.