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Advice 2 min

Last Updated Jan 31, 2024

What does SPF level mean?

Author
|
Dermatology Resident
Dr. Paarth Dodia

SPF is an acronym for “Sun Protection Factor” and grades the strength of a sunscreen at blocking harmful UVB rays. 

UV light from the sun is divided into two categories, UVA and UVB. UVA rays have longer wavelengths, while UVB rays have shorter wavelengths. UVA radiation penetrates deeper into the skin’s layers than UVB, damaging collagen, which is responsible for wrinkles and other signs of aging. UVB doesn't penetrate as deep but it directly damages DNA leading to various forms of skin cancer. SPF stands for “Sun Protection Factor” and measures how well sunscreen can specifically protect your skin from UVB radiation. 

When you apply sunscreen with a higher SPF number, it means you'll get more protection. SPF value is calculated in the lab by first exposing skin to UV radiation and determining the minimum amount of energy needed to produced a sunburn. This is called the minimum erythematous dose or MED. The SPF value is then calculated based on the ratio of MED of protected skin (under the sunscreen) and MED of unprotected skin (without sunscreen).

SPF 15 Blocks 93% of UVB
SPF 30 Blocks 97% of UVB
SPF 50 Blocks 98% of UVB

Remember, that the SPF is tested at 2mg/cm2, which means if you want to get the strength on the bottle, you should use the same amount. For your head and neck, a rough estimate is two finger lengths of sunscreen (or ¼ teaspoon). Don’t forget to use other sun protection measures like wearing a hat, sunglasses, protective clothing, and seeking shade when the sun is the strongest.

(Updated after errata submitted by user on January 31st, 2024)

Doctor's Note

SPF stands for Sun Protection Factor and it's a calculation of how effective your sunscreen is against UVB rays

Aim for minimum SPF 30

Use two finger lengths of sunscreen for your head and neck to achieve adequate sun protection

The Research

Williams JD, Maitra P, Atillasoy E, Wu MM, Farberg AS, Rigel DS. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018;78(5):902-910.e2. doi:10.1016/j.jaad.2017.12.062

PMID: 29291958

Dr. Paarth Dodia

Dermatology Resident

Dr. Paarth Dodia

Dermatology Resident

Dr. Paarth Dodia grew up in the big state of Texas. Currently, he is completing his dermatology residency in Wilmington, North Carolina. Dr. Dodia is all about spreading the love for dermatology. He finds so much joy in helping people reveal their radiant, glowing skin in scientifically proven ways. When Dr. Dodia isn't at the clinic, you'll catch him happily strumming on a guitar, whipping up delicious meals in the kitchen, and staying physically active to keep fit.

Doctor's Note

SPF stands for Sun Protection Factor and it's a calculation of how effective your sunscreen is against UVB rays

Aim for minimum SPF 30

Use two finger lengths of sunscreen for your head and neck to achieve adequate sun protection

The Research

Williams JD, Maitra P, Atillasoy E, Wu MM, Farberg AS, Rigel DS. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018;78(5):902-910.e2. doi:10.1016/j.jaad.2017.12.062

PMID: 29291958

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