Safety of Oxybenzone: Putting Numbers Into Perspective FREE

Oxybenzone, an organic UV-B and short-wave UV-A filter, has been available for over 40 years¹; it is widely used in sunscreens and other consumer products in the United States.² The Centers for Disease Control and Prevention has estimated the prevalence of oxybenzone exposure in the general US population to be 96.8%.³ In the past few years, oxybenzone has received increasing attention as a potentially harmful compound. Initial concerns arose when a report demonstrated systemic absorption of oxybenzone in humans at a rate of 1% to 2% after topical application.⁴ Similar or higher rates of cutaneous absorption in human subjects have been observed.^5- 9 The potential for biological effects, however, were first published in a study by Schlumpf et al¹⁰ demonstrating uterotropic effects in immature rats after oral administration of oxybenzone; it should be noted that the estrogenic effect detected was less than 1 million-fold of estradiol, the positive control used. Nonetheless, this study has served as the basis for considerable concern among the public.

In assessing the potential for hormonal disruption in humans, we decided to place into perspective the doses of oxybenzone used by Schlumpf et al¹⁰ to achieve the 23% increase in uterine size reported in immature rats. We performed 2 calculations: (1) we determined the equivalent amount of sunscreen required to be used topically in humans to achieve the effective cumulative amount of oxybenzone orally administered to immature rats; and (2) we determined the number of years of daily application required to obtain the equivalent levels of oxybenzone that the experimental animals were exposed to.

The oral dose of oxybenzone used by Schlumpf et al¹⁰ was 1525 mg/kg/d over a 4-day period. The effective cumulative dose was 6100 mg/kg. To calculate an equivalent amount of sunscreen, the following assumptions were applied to the formula: (1) the weight of an average woman in the United States was assumed to be 74.6 kg¹¹; (2) the absorption rate of topically applied oxybenzone, 10%, was assumed to be approximately 2%^4- 9; and (3) the maximum concentration of oxybenzone in sunscreen sold within the United States was assumed to be 6% (wt/vol), or 60 mg/mL.¹²

([6100 mg of Oxybenzone/kg   ×  74.6 kg]/2%)   ×  (1 mL of Sunscreen/60 mg of Oxybenzone)  =  379  217 mL

We then calculated the number of years of daily application that would be required to apply 379  217 mL of sunscreen under 3 different scenarios:

Scenario 1: 100% body surface area (BSA) coverage at a standard dose of 2 mg/cm² would require 30 mL (10  950 mL/y with daily application).

Scenario 2: 100% BSA coverage at a dose of 1 mg/cm² would requires 15 mL (5475 mL/y).

Scenario 3: 25% BSA coverage at a dose of 1 mg/cm² would require 3.75 mL (1369 mL/y).

For the purposes of this estimation, a generous in-use dose of 1 mg/cm² was used,^13- 16 and it was assumed that coverage of 25% of the human BSA was limited to the face, neck, hands, and arms.¹⁷

The numbers of years of daily application required to obtain the equivalent amount of sunscreen under the 3 scenarios are listed in the Table.

Our results indicate that both the application regimens and time periods required to obtain systemic levels of oxybenzone equivalent per unit of body mass are essentially unattainable. Our assumption is more conservative. For instance, the bioavailability of the oxybenzone-containing rat chow used by Schlumpf et al¹⁰ may not have been 100%, and we did not take into account the excretion of oxybenzone in humans. In fact, oxybenzone has not been demonstrated to accumulate in the plasma even after several days of topical application.^6,9,18 Most relevant to this discussion, however, is that in a human study, oxybenzone did not demonstrate significant endocrine disruption, even with application of a formulation containing 10% oxybenzone.⁶ In fact, after 40 years of use, we are not aware of any published study that demonstrates acute toxic effects in humans with systemic absorption of oxybenzone.

We do not intend for this exercise to serve as a basis from which to legitimize the extrapolation of data from immature rats to humans. In fact, basic scientific principles regarding the complexities of each respective biological system preclude this. Nonetheless, we hope that this analysis helps to place into perspective the doses reported by the in vivo study from which inappropriate conclusions have been drawn and considerable controversy has developed.