SPF is an acronym for Sun Protection Factor and it is a calculation based on how long it takes for someone to be over exposed to UVB rays while wearing sunscreen compared to someone who is not.
Many people think that it represents the number of hours that you can stay in the sun if you've applied the sunscreen. Sadly, this idea may account for the unexpected number of sun related skin cancers that many countries have recently experienced. The sunscreen itself can give a false sense of protection and unknowingly, people subject themselves to harmful UVA and UVB rays.
How to use the SPF as a practical guide:
The calculation is not that difficult. It is basic multiplication: Your skin type x SPF = Maximum Protection Minutes.
For example, a fair skinned person who burns after 10 minutes without a sunscreen will be protected for 60 minutes if he or she uses a sunscreen with an SPF of 6 (10 minutes x 6 SPF = 60 minutes of protection).
Assuming that you have applied the right amount of sunscreen, below are the MAXIMUM minutes that you could expect to be safely in the sun, based on your skin type and SPF.
Light to Fair Skin
Without Sunscreen the SPF value is zero and you can expect to start feeling over exposed after 10 minutes.
SPF 6: 60 minutes (1 hour)
SPF 15: 150 minutes (2.5 hours)
SPF 30: 300 minutes (5 hours)
Without Sunscreen the SPF is zero and the maximum number of minutes is 15.
SPF 6: 90 minutes (1.5 hours)
SPF 15: 225 minutes (3.75 hours)
SPF 30: 450 minutes (7.5 hours)
Without Sunscreen the SPF is again zero and the maximum time is 20 minutes.
SPF 6: 120 minutes (2 hours)
SPF 15: 300 minutes (5 hours)
SPF 30: 600 minutes (10 hours)
Don't be seduced by these numbers.
Remember that real life is not a mathematical formula. How long you can be exposed to the sun depends on:
What is your skin type and how sensitive it is to the sun?
Have you applied sunscreen in the right amounts?
Are you engaged in any physical activity that would result in the sunscreen being thinned out, washed or sweated away?
Another thing to consider is that most people do not apply the sufficient amount of sunscreen. If so, the total time is reduced significantly. For optimal protection, apply sunscreen 15 to 30 minutes before you are in the sun and reapply after any physical activity.
The UV Index represents the intensity of ultraviolet exposure at the sun's highest point in the day, which is generally between 11:30 am and 1:30 pm. For example, this is what New York City looks like: (Credit)
Media graphic color
Low danger from the sun's UV rays for the average person
Wear sunglasses on bright days; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
Moderate risk of harm from unprotected sun exposure
Take precautions, such as covering up, if you will be outside. Stay in shade near midday when the sun is strongest.
High risk of harm from unprotected sun exposure
Wear sunglasses and use SPF 30+ sunscreen, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun within three hours of solar noon.
Very high risk of harm from unprotected sun exposure
Wear SPF 30+ sunscreen, a shirt, sunglasses, and a hat. Do not stay in the sun for too long.
Extreme risk of harm from unprotected sun exposure
Take all precautions, including: wear sunglasses and use SPF 30+ sunscreen, cover the body with a long-sleeve shirt and trousers, wear a very broad hat, and avoid the sun from three hours before until three hours after solar noon.