In a 2021 study by Biomedical Optics Express, researchers used a mouse model of human skin tumors. It confirmed that LEDs with a 642 nm, which falls within the red light slot, caused an increase in tumor size. It also increases epidermal thickness and the number of inflammation-causing proteins called cytokines.
This study highlights the
possibility of red light causing cancer cell growth. But it isn’t definitive.
Another 2022 study used in vitro (cells in petri dishes) and animal models (whole living organisms).
Note: J/cm² RL measures the energy density of red light applied to a given area.
In vitro, when 640 J/cm² of red light was used, it decreased cellular growth. It did so without boosting the rate of old cell tissue removal (apoptosis). When 1280 J/cm² was applied to the cells, the apoptosis rate increased.
In the animal models, 2560 J/cm² red light prevents melanoma growth. At 1280 and 1920 J/cm² RL, the tumor volume reduced, but not significantly.
The conclusion was that RL is a potentially safe and effective therapy for melanoma. It prevents tumor growth and boosts immunity to fight off the cancer cells. Still, more studies are needed. 
