Why spf 50 give most people an spf of only 2.7

"It turns out that if you apply half the amount, you get the protection of only the square root of the SPF," said Dr. Darrell S. Rigel, a clinical professor of dermatology at New York University, who has done efficacy testing for Johnson & Johnson and the Procter & Gamble Company.

So applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4, Dr. Rigel said.

http://www.nytimes.c.../...nted=1&_r=1


So, how much do you apply and what SPF do you really get?


This is from a graph in Pinnell, Cutaneous photodamage, oxidative stress, and topical antioxidant protection, J Am Acad Dermatol, Jan 2003

applying 2 mg/cm^2 (about 0.37 tsp to face)

spf ; true spf
2 ; 2
4 ; 4
8 ; 8
15 ; 15
30 ; 30
50 ; 50

applying 1.5 mg/cm^2 (about 0.28 tsp to face)

spf ; true spf
2 ; 1.7
4 ; 2.8
8 ; 4.8
15 ; 7.6
30 ; 12.8
50 ; 18.8

applying 1.0 mg/cm^2 (about 0.19 tsp to face)

spf ; true spf
2 ; 1.4
4 ; 2.0
8 ; 2.8
15 ; 3.9
30 ; 5.5
50 ; 7.1


applying 0.5 mg/cm^2 (about 0.1 tsp to face, most typical)

spf ; true spf
2 ; 1.2
4 ; 1.4
8 ; 1.7
15 ; 2.0
30 ; 2.3
50 ; 2.7

(modified from Wulf et al., J Photoderm Photoimmun Photomed 1997;13:129-132)

Edited by Fredrik, 14 May 2009 - 10:28 AM.

Damn, I probably apply way too little. 0.37 tsp sounds like a lot.

Click HERE to rent this advertising spot for AGELESS LOOKS to support LongeCity (this will replace the google ad above).

I have read that you need to apply 1/4 teapoon for the face and neck. For a combined 1/2 teaspoon for both.

Is this amount just for the face and not the neck?
applying 2 mg/cm^2 (about 0.37 tsp to face)

If it is just for the face, it seems a little higher than the popular 1/4 teaspoon.

Just curious.

"It turns out that if you apply half the amount, you get the protection of only the square root of the SPF," said Dr. Darrell S. Rigel, a clinical professor of dermatology at New York University, who has done efficacy testing for Johnson & Johnson and the Procter & Gamble Company.

So applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4, Dr. Rigel said.

http://www.nytimes.c.../...nted=1&_r=1


So, how much do you apply and what SPF do you really get?


This is from a graph in Pinnell, Cutaneous photodamage, oxidative stress, and topical antioxidant protection, J Am Acad Dermatol, Jan 2003

applying 2 mg/cm^2 (about 0.37 tsp to face)

spf ; true spf
2 ; 2
4 ; 4
8 ; 8
15 ; 15
30 ; 30
50 ; 50

applying 1.5 mg/cm^2 (about 0.28 tsp to face)

spf ; true spf
2 ; 1.7
4 ; 2.8
8 ; 4.8
15 ; 7.6
30 ; 12.8
50 ; 18.8

applying 1.0 mg/cm^2 (about 0.19 tsp to face)

spf ; true spf
2 ; 1.4
4 ; 2.0
8 ; 2.8
15 ; 3.9
30 ; 5.5
50 ; 7.1


applying 0.5 mg/cm^2 (about 0.1 tsp to face, most typical)

spf ; true spf
2 ; 1.2
4 ; 1.4
8 ; 1.7
15 ; 2.0
30 ; 2.3
50 ; 2.7

(modified from Wulf et al., J Photoderm Photoimmun Photomed 1997;13:129-132)

Thank you for posting this interesting article, Fredrik!
Once you recommended in one of your posts of two applications of sunscreen so all areas would be perfectly covered. I think it is also an excellent way to ensure that the amount one applies normally would double hence giving a higher SPF and UVA value than only one application would. (Even though 2 X 0.1 tsp. of SPF 50 will only give a bit over SPF 7 according to this study. Thenone would wonder of course how much UVA protection one would get even from an EU sunscreen that is well formulated. PPD 2?!)

Edited by Eva Victoria, 15 May 2009 - 02:27 PM.