D. R. Thomas, MD, FRCPC
Faculty of Medicine, University of British Columbia, Vancouver, Canada
The skin has evolved to protect us from the harmful effects of ultraviolet light. Sunscreens were first developed to prevent sunburns by blocking UVB; they allowed us to prolong our time in the sun, but that resulted in increased exposure to UVA. Most modern sunscreens
attempt to block the whole spectrum of UV light, however not all so-called broad spectrum sunscreens protect skin from the whole range of UVA.
A Comparison on UVA and UVB
|Comparisons||UVA (320-400nm)||UVB (290-320nm)|
|Levels||Levels are constant throughout the year||Amounts vary and increase in the summer, at noon, and on the equator|
|Penetration||Penetrates into the lower dermis||Most only penetrates the epidermis|
|Levels through glass||Penetrates glass||Does not go through glass|
|Other||95% of UVL is UVA||SPF of sunscreens only measures UVB blockage|
|Table 1: Comparison of UVA and UVB light|
|Effects on the Skin||UVA*||UVB|
|Carcinogenic level||May be important in causing melanoma||More carcinogenic than UVA|
|Changes to the skin||Sunburns the skin|
|Systemic Effects||Immunosuppressive||Needed for Vitamin D production|
|Table 2: Effects on the skin.|
*It should be noted that sun-tanning beds use mainly UVA light. There is no such thing as a “safe” suntan. Recently the US FDA began investigating whether suntan beds should be illegal for anyone under the age of 18 years.
Long-term Effects on the Skin
Most of us know that sun exposure has immediate positive and negative effects on the skin. The medium and longer term effects are negative. Positive effects include a sense of warmth, pleasure and Vitamin D production.
|Short-term effect||Medium-term effect||Long-term effect|
|Negative Effects:||Negative Effects:||Negative Effects:|
These occur only when activated by UV light. Most of them are brought on by UVA. Photosensitive rashes (detailed below) can be thought of as:
1 – Idiopathic reactions to UV light
(Polymorphous light eruption – PLE)
The timing of the onset of the rash in relation to sun exposure and its duration, as well as the type of reaction produced on the skin, is key to making the diagnosis. As always there is some variation.
2 – Phototoxic and photoallergic skin reactions
There are a number of drugs and chemicals that can produce a reaction in the skin. These can be either phototoxic or photoallergic reactions.
3 – UV aggravation of existing conditions
There are many pre-existing conditions that are aggravated by sunshine. Some of the important light aggravated conditions are:
Polymorphous Light Eruption
|Sun Exposure Causes|
|Polymorphous light eruption (PLE)||Treatment of PLE|
Photoxic vs. Photoallergic Reactions
Phototoxic drugs or chemicals
- Usually seen within hours
- Usually caused by UVA
Includes tetracyclines, sulfa, amioderone, fursomide, naproxen, piroxicam, chlorpromazine, ciprofloxacin, thiazides
- An eczema like reaction. Can be thought of as a delayed hypersensitivity type reaction.
- Causes: Sunscreens, fragrances/aftershave (like musk ambrette, sandalwood oil), chlorhexidine
- A special type of reaction to topical contact with a sensitizer called psoralen contained in a number of plants.
- UVA plus psoralen will produce a blistering reaction often seen in streaks; a brown pigmentation is produced
- which may last for months.
- Plants containing psoralen are responsible including lime, yarrow, cow parsley, celery, lemon, fig
Should be encouraged to prevent the immediate, medium and long-term ill effects of excessive sun exposure. Some sun exposure is desirable for vitamin D production.
Two ways to encourage photoprotection:
1. Sun avoidance
- Avoid the sun between 10am and 3pm.
- Try to stay in the shade.
- Wear protective, tightly woven clothes and a broad brimmed hat.
2. Sun protection
- Use a Broad-spectrum sunscreen in a sufficientquantity.
- SPF = the ratio of minimal erythema dose (MED) of protected skin/MED of unprotected
- skin. This is a crude biological measure.
- The SPF factor is calculated using 2mg/cm2 of sunscreen. Most people apply only 25-50% of this.
- Reapply sunscreen every 2 hours; UVL causes some chemical sunscreens to become inactive over time.
- Very popular, producing good, even colour.
- Contain dihydroxyacetone (DHA); reacts with amino acids containing keratin. DHA concentration varies from 2%-6%; higher numbers give a darker colour.
- DHA has an SPF of 2%-3%. Some have a low SPF screen added that lasts only a few hours.
- Coloured skin does not provide protection against photodamage.
- Bronzers are dyes that are added to the skin; can be washed off.
- Beta-carotene, tyrosine, tanning accelerators such as psoralen are not recommended.
- Broad spectrum only should be used.
- SPF related to UVB protection only; does not provide a reference to UVA protection.
- All sunscreens have UVB protection; reflected in the SPF.
- If skin sunburns in 10 minutes, properly applied sunscreen at SPF 15 means skin will burn in 150 minutes.
- Physical screens reflect light; chemical screens absorb UV, converting energy into heat
- SPF15 blocks 87.5% of UVB and SPF 50 blocks 98% of UVB.
Sunscreen with full spectrum UVA protection contains:
Avobenzone (Parsol 1789), Mexoryl Sx, and Zinc oxide working together. The first two have slightly different peaks of protection. Titanium Dioxide, Dioxybenzone Methyl anthranilate and Octocrylene provide UVA protection, but not along the whole spectrum. Some recommended general sunscreens: Ombrelle® 30, 45, 60, cream and lotion. (This broad spectrum sunscreen was pioneered in Canada); Anthelios® 30, 45, 60; Neutrogena® Heathy Defense Sunblocks 30, 45 with parsol.
Lip protection: SCC is more commonly seen in men and women who don’t wear lipstick:
Ombrelle® Lip Balm SPF30; RoC Minesol® Lipstick SPF 20; Neutrogena Stick 30; Antherpos ® SPF 50. For joggers these can also be used above the eyebrows to prevent the screen from entering the eyes. Can also be used on the nose.
Spray for athletes, or for people with hairy or oily skin: Ombrelle® Sport Spray 15; Coppertone ® Sport 15 and 30; Neutrogena® Healthy Defense Spray 30.