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
  • Tans the skin
  • Causes most of the aging effects seen in the skin
Sunburns the skin
Systemic Effects Immunosuppressive Needed for Vitamin D production
Other effects
  • Phototoxic reactions to drugs and chemicals
  • Responsible for many photodermatoses
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:
  • Sunburn, or tan
  • Photosensitive rashes
  • Drug and chemical phototoxicity and allergy reactions
  • Light aggravated conditions
Negative Effects:
  • Photodamage
  • Photoaging
Negative Effects:
  • Skin cancer
  • Photoaging

Photosensitive Rashes

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:

  • Rosacea
  • Melasma
  • Dermatomyositis
  • Viral Exanthem
  • Darier’s disease
  • Systemic lupus erythematosus (SLE)
  • Seborrehic dermatitis
  • Discoid lupus
  • Herpes simplex
  • Pemphigus
  • Porphyrias


Polymorphous Light Eruption

Sun Exposure Causes
  • Rash starts within hours of exposure and lasts for days even with no further sunshine. Solar urticaria is seen within minutes of exposure.
  • Rash can be seen in the form of papules, papulovesicles or plaques, hence the term polymorphous.
  • Plaques are less commonly seen.
Polymorphous light eruption (PLE)
  • Mostly caused by UVA
  • Occurs in early spring or summer, and often during vacation periods
  • Is mostly symmetrical, red papules and plaques.
  • Occurs in exposed areas but not necessarily all the exposed areas
  • Occurs in 10%-20% of the population
  • May be confused for allergic reaction to sunscreen
  • Skin tends to be less reactive to sun as the summer progresses.
  • The type of rash tends to remain constant for each patient.
Treatment of PLE
  • Sun avoidance and protection with broad spectrum sunscreen (UVA and UVB)
  • Topical steroids and antihistamines
  • UVA and UVB light therapy may help some at the start of the season, which can harden the skin and prevent the reaction.
  • Hydroxychloroquine, 400mg daily for 2 weeks in the spring or before a vacation may help

Photoxic vs. Photoallergic Reactions

Phototoxic drugs or chemicals

  • Sunburn-like
  • Usually seen within hours
  • Usually caused by UVA

Includes tetracyclines, sulfa, amioderone, fursomide, naproxen, piroxicam, chlorpromazine, ciprofloxacin, thiazides

Photoallergic reactions

  • 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.

Sunbed Tan

  • 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.

Sunscreen Use

  • 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 Choices

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.

Special sunscreens:
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.