According to a survey conducted by Kao, 49% of 1,500 respondents reported experiencing a ‘tingling’ and ‘burning’ of skin exposed to strong sunlight.
The company’s Skin Care Research Laboratory and Processing Development Research Laboratory narrowed down the cause of the skin surface temperature increase to near-infrared (NIR) rays.
Solar light can be roughly categorised based on wavelength, such as ultraviolet, visible, NIR rays.
While the photo-aging and inflammatory effects of UV ray exposure are well-known, NIR has been reported to induce oxidative stress, wrinkles, and skin sagging, as well as increased skin surface temperature.
Kao claimed that a conventional sun screen formulation that only contain UV ray protective agents could not prevent this rise in skin surface temperature.
Through more focused studies, Kao researchers found that NIR was the cause of the burning sensations.
They confirmed that the skin surface would elevate by 5 to 6 degrees Celsius within approximately five minutes of exposure to direct sun rays.
They also found that the skin’s temperature can go up to 40 degrees Celsius when the surrounding temperature exceeds 28 degrees Celsius. When the skin’s surface increases to around 42 degrees Celsius, its pain receptors get activated and causes the sensation of pain.
Thermal control
The team investigating a number of materials that can potentially block NIR by light scattering.
It discovered that titanium dioxide flakes had high NIR protection capabilities and it could be controlled by adjusting the thickness of the flakes.
A test was carried out to determine the efficacy of the titanium flakes. Researchers compared NIR protective material combined with UV protection formulation and a conventional sunscreen formulation on forearms.
Following exposure to artificial sunlight, changes in skin surface temperature were determined.
The average skin temperature of forearms without sun screen application was elevated by 6.5 degrees Celsius after five minutes of exposure, while the forearms protected by the conventional sunscreen formulation also increased in a similar manner.
In contrast, the skin surface shield by UV and NIR protective ingredients saw a decrease by an average of -1 degrees Celsius and a maximum of -1.6 degrees Celsius.
Self-evaluations were also carried our five minutes after sun exposure. Through a visual analogue scale, subjects that received the UV and NIR application on the forearm reported feeling significantly less of the burning sensation associated with sun exposure.
The researcher concluded that this new technology could reduce skin surface temperature increase and the sensation of heat caused by sun exposure.
“The findings obtained will be implemented for further development of sunlight protective care technology,” said the company.
The findings of this study were presented at the 85th meeting of the Society of Cosmetic Chemists of Japan (SCCJ) in November 2019 where it won the best presentation award.
