If you’ve ever been to the beach, played an outside sport, or taken a walk on a hot day, chances are that you’ve used sunscreen. Sunscreen is an integral component of safe outdoor recreation as shields our skin from the sun’s harmful ultraviolet (UV) rays.
UV rays are a type of high-frequency electromagnetic radiation that can damage living tissue. UVB and UVA rays are the two types of ultraviolet rays that seep through the ozone layer and reach the earth. Both UVB and UVA rays can inflict irreparable harm on human health. UVB rays generate sunburns and skin cancers, including the potent malignant melanoma. UVA rays likewise contribute to the development of skin cancer. They penetrate the skin, reaching the dermis layer and inducing premature skin aging, such as wrinkles. UVA wavelengths are longer than UVB wavelengths, therefore a greater amount of the former can pass through the ozone layer. As a result, UVA rays vastly outnumber their UVB counterparts by a ratio of approximately 500:1.
Sunscreen is a vital means of defense against the threats UV rays pose. The two types of sunscreens are physical blockers and chemical blockers. Physical blockers reflect UV rays from the sun, while chemical blockers absorb them.
Physical blockers contain zinc oxide and titanium dioxide, organic chemicals that form a protective layer on the skin. Zinc oxide is an insoluble mineral that reflects and scatters UV rays, repelling both UVA and UVB rays. Titanium dioxide scatters and absorbs UV radiation. However, the average range of reflection for zinc oxide and titanium dioxide is only 4-5%. The rest of the UV protection in physical blockers is provided by a semiconductor band gap that absorbs UV photons. Zinc oxide and titanium dioxide are semiconductors, meaning that their conductivity ranges between that of an insulator and that of a metal. Electrons are concentrated in bands of energy in semiconductors and insulators. The energy difference between the top of the valence band and the bottom of the conduction band is referred to as the band gap. UV rays whose wavelengths exceed the band gap absorption energy levels are reflected. If the wavelength of a ray is equal to or smaller than the band gap energy absorption energy level, the ray is absorbed. Titanium dioxide has a band gap energy of 3.06 eV that corresponds to a wavelength of 412.5 nm. “Light at or below this wavelength will have enough energy to excite electrons from the valence band to the conduction band”, endocrinologist Dennis Dudley said. “Any photon with a wavelength longer than the band gap will not be absorbed by the sunscreen”. The maximum wavelengths of UVB and UVA are 380 nm and 400 nm, respectively. Therefore, the titanium dioxide band gap would be able to absorb both rays. The energy absorption level of zinc oxide is 3.37 eV at room temperature, which corresponds to 375 nm. The wavelengths of UVA and UVB rays could potentially exceed 375 nm, resulting in their reflection.
There are several benefits to physical blockers. They cover a broad spectrum of wavelengths, shielding the skin from both UVA and UVB rays. The sunscreen blocks UV rays immediately upon being applied. Physical blockers also last longer than chemical blockers in direct UV light, have a longer shelf life, and they are less likely to irritate sensitive skin. The cons of physical blockers are few. Their thick texture makes the sunscreen difficult to blend. As a result, the sunscreen’s pale color can starkly contrast against darker skin tones. Moreover, physical blockers are not adhesive and wash off with little effort.
Chemical blocker sunscreens seep into the skin and absorb UV rays. The UV rays are converted to heat ad released from the body. Chemical blockers mainly consist of the active agents avobenzone, octinoxate, and oxybenzone. Once infused into flesh, the inorganic chemicals interact with the epidermis to absorb UV rays through their chemical bonds. While the bonds intake UV radiation, the components of the chemical blocker disintegrate and dissipate heat. Each agent filters different ranges on the spectrum of UV rays. Together, the inorganic chemicals cover the entire spectrum.
Chemical blockers are the most popular sunscreens because of their blendability, thin texture, and ability to be sold in multiple states, ranging from creams and sprays. But, unfortunately, the ingredients used in chemical blockers are harmful to both human health and the environment. Earlier this year, a study published in the medical journal JAMA revealed that key ingredients of chemical blockers “enter[ed] the bloodstream at levels that far exceed the FDA's recommended threshold without a government safety inspection”. In the study, 24 participants applied four different types of sunscreen four times per day. Afterward, the researchers measured the concentration of the four active ingredients in the participants' bloodstream. The ingredients were avobenzone, oxybenzone, octocrylene, and ecamsule. If the concentration surpassed 0.5 ng/ml, the FDA advises “nonclinical toxicology assessment including systemic carcinogenicity and additional developmental and reproductive studies”. The level of absorption of all agents exceeded the limit on the first day of study by a large margin. Three of the ingredients flowed in the participants’ blood for an entire seven days. Oxybenzone was the most persistent. “Concentrations reached the threshold within two hours after a single application and exceeded 20 ng/mL on day 7 of the study”, USA Today reports.
Other studies show that high exposure to oxybenzone disrupts hormones and reduces fertility. “Animal studies report lower sperm counts and sperm abnormalities after oxybenzone and octinoxate exposure; delayed puberty after octinoxate exposure; and altered estrous cycling for female mice exposed to oxybenzone”, the Environmental Working Group states. “Recently, Danish researchers reported that eight of 13 chemical sunscreen ingredients allowed in the U.S. affected calcium signaling of male sperm cells in laboratory tests, which the researchers suggest could reduce male fertility”. Human male exposure to oxybenzone likewise produced poor-quality sperm. Plus “female exposures to oxybenzone and related chemicals have been linked to increased risk of endometriosis”.
The ingredients of chemical blockers carry an environmental cause as well. In May 2018, Hawaii banned sunscreens containing oxybenzone and octinoxate because they were damaging coral reefs. Every year, 14,000 tons of sunscreen end up in coral reefs. The bill states that oxybenzone and octinoxate “cause mortality in developing coral; increase coral bleaching that indicates extreme stress, even at temperatures below 87.8 degrees Fahrenheit; and cause genetic damage to coral and other marine organisms”.
Applying sunscreen is vital to shielding our skin against the sun’s harmful rays. However, we must ensure that we aren’t compromising the environment and other aspects of our health in the process. Current research clearly indicates that physical blockers are the most effective and safe means of protection. The next step is to popularize physical blockers and educate the public about the dangers of their chemical counterparts.