Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.See all posts »
Are Sunscreens Effective?
Nearly a year ago, there was a news item that suggested that many sunscreens are ineffective, despite manufacturers' claims to the contrary. As best I can tell, the issues are yet to be resolved.
What are the issues? These relate to the safety of the chemicals contained in the sunscreens, what comprises an optimal application (volume, thickness, substantivity, etc.) of suncreens, how frequently sunscreens should be applied, their comparative efficacy compared to occlusive clothing, and their inherent sunscreening properties.
According to last year's report, the products of, among others, Copppertone, Banana Boat, and Neutrogena might be deficient in one way or another. Of course, the companies mentioned denied any shortcomings.
The Environmental Working Group's report is still available on the Internet.
Regardless of whether one believes every opinion presented in the report, there are still some truths that seem evident, such as the fact that no sunscreen can be completely protective, so that if a person is at a very high risk for skin cancer, a clothing barrier (to ultraviolet radiation) makes the most sense. Furthermore, it's highly unlikely that any sunscreen performs as well as advertised, either because its application has been imperfect, the product breaks down in sunlight and other environmental conditions and loses effectiveness, or the product is washed off.
Sun Exposure Basics
Here is some information about sun exposure that I will be offering in the forthcoming 5th edition of the book Medicine for the Outdoors. It assumes that manufacturers of sunscreen products are making true assertions and claims.
The solar radiation that strikes the earth includes 50% visible light (wavelength 400 to 760 nanometers), 40% infrared (760 to 1,700 nm), and 10% ultraviolet (UV) (10 to 400 nm). Sunburn is a cutaneous photosensitivity reaction caused by exposure of the skin to ultraviolet radiation (UVR) from the sun. There are four types of UVR: vacuum UVR is 10 to 200 nm (absorbed by air and unable to penetrate Earth’s atmosphere), UVA is 320 to 400 nm, UVB is 290 to 320 nm, and UVC is 100 to 290 nm. UVC is filtered out by the ozone layer of the atmosphere. UVB is the culprit in the creation of sunburn and cancer. UVA is of less immediate danger but is a serious cause of skin aging, drug-related photosensitivity, and skin cancer.
Ultraviolet exposure varies with the time of day (greatest between 9 A.M. and 3 P.M. because of increased solar proximity and decreased angle of light rays), season (greater in summer), altitude (8 to 10% increase per each 1,000 ft, or 305 m, of elevation above sea level), location (greater near the equator), and weather (greater in the wind). Snow or ice reflects 85% of UVR, dry sand 17%, and grass 2.5%. Water may reflect 10 to 100% of UVR, depending upon the time of day, location, and surface. However, UVR at midday may penetrate up to 24 in (60 cm) through water. Clouds absorb 10 to 80% of UVR, but rarely more than 40%. Most clothes reflect (light-colored) or absorb (dark-colored) UVR. A dry white cotton shirt has a maximum SPF of 8 (see Sunscreens, below). However, it is important to note that wet cotton of any color probably transmits considerable UVR.
People may be more sensitive to UVR after they have ingested certain drugs (such as tetracycline, doxycycline, fluoroquinolones, vitamin A derivatives, non¬steroidal anti-inflammatories, sulfa derivatives, minoxidil, diltiazem, nifedipine, thiazide diuretics, hypoglycemic agents, chloroquine, dapsone, quinidine, carbamazepine, chemotherapeutic drugs, and barbiturates) or have been exposed to certain plants (such as lime, citron, bitter orange, lemon, celery, parsnip, fennel, dill, wild carrot, fig, buttercup, mustard, milfoil, agrimony, rue, hogweed, Queen Anne’s lace, and stinking mayweed). Your eyes may become more sensitive to light (e.g., you may need to wear sunglasses at a lower ultraviolet threshold) if you are taking certain medications, such as digitoxin, quinidine, tolazamide, or tolbutamide.
Sunscreens (available as lotions or creams) either absorb light of a particular wavelength, act as barriers, or reflect light. There is no evidence that any ingredients in sunscreens cause skin damage or cancer. Choose sunscreens based on your estimated exposure and on your own propensity to tan or burn. There is no such thing as a “safe tan,” even when sunscreens are used, because sun exposure is directly linked to skin cancer. In addition, long-term exposure to ultraviolet radiation from sunlight causes premature skin aging and loss of skin tone.
Dermatologists classify sun-reactive skin types (based on the first 45 to 60 minutes of sun exposure after winter or after a prolonged period of no sun exposure) as follows:
Type I. Always burns easily, never tans. (Fair-skinned people with a high number of moles are at the greatest risk for melanoma.)
Type II. Always burns easily, tans minimally.
Type III. Burns moderately, tans gradually and uniformly (light brown).
Type IV. Burns minimally, always tans well (moderate brown).
Type V. Rarely burns, tans profusely (dark brown).
Type VI. Never burns, is deeply pigmented (black skin).
In all cases it is wise to overestimate the protection necessary and to carry a strong sunscreen. To protect hair from sun damage, wear a hat.
Para-aminobenzoic acid (PABA) derivatives, which are water soluble, are sunscreens that absorb UVB (not UVA) and that accumulate in the skin with repeated application. The most commonly used PABA derivative is padimate O (octyl dimethyl PABA). The most effective method of application is to moisturize the skin (shower or bathe) and then apply the sunscreen. For maximum effect, chemical sunscreens should be applied liberally (most people only apply ¼ to ½ of what they need) at least 15 to 30 minutes prior to exposure, and the skin should be kept dry for at least 2 hours after sunscreen application. Sun blockers, such as titanium, are effective essentially immediately. When PABA itself is used, a recommended preparation is 5 to 10% PABA in 50 to 70% alcohol. However, PABA is now used infrequently because its absorption peak of UVB at 296 nm is too far from 307 nm, where UVB exerts its greatest effect. Furthermore, it causes skin irritation—a stinging sensation—and can stain cotton and synthetic fabrics. PABA derivatives are less problematic.
Benzophenones are sunscreens that are more effective against ultraviolet A. These should be used in 6 to 10% concentration. Because they are not well absorbed by the skin, they require frequent reapplication. Photoplex broad-spectrum sunscreen lotion contains a PABA-ester combined with a potent UVA absorber, Parsol 1789. This is an excellent sunscreen for sensitive people, particularly those at risk for drug-induced photosensitivity.
SPF and UVA/UVB Protection
Sunscreens come in different concentrations (such as PreSun “8” or “15”). A higher sun protection factor (SPF) number (range 2 to 50) indicates a greater degree of protection against UVB. “Minimal erythema dose” (MED) is the amount of UVR exposure required to redden the skin. SPF is derived by dividing the MED of skin covered with sunscreen by the MED of unprotected skin. Thus, an SPF of 15 indicates that it requires 15 times the UVR exposure to achieve a sunburn as it would without protection. The SPF number assumes a liberal (approximately 11/4 oz, or 37 ml, per adult) application of the sunscreen.
There is no standard for measuring UVA protection. Persons with sensitive or unconditioned skin should use a sunscreen with an SPF number of 10 or greater. Fair-skinned people who never tan or who tan poorly (Types I, II, or III) or mountain climbers (there is more UV exposure at higher altitudes, and more is reflected off snow) should always use a sunscreen with an SPF number of 15 or greater. Most sun exposure occurs prior to the age of 18 years, so it is very important to apply sunscreens to children and young adults.
Substantivity refers to the ability of a sunscreen to resist water wash-off. Layering sunscreens doesn’t work well, because the last layer applied usually washes off.
Proper Sunscreen Application
Sunscreens are first applied to cool, dry skin for optimal absorption; wait 10 minutes prior to water exposure. Reapply them liberally after swimming or heavy perspiration. In general, most sunscreens should be reapplied every 20 minutes to 2 hours. Be aware that the concomitant use of insect repellent containing DEET lowers the effectiveness of the sunscreen by a factor of one-third.
Although many sunscreens are designed to bond or adhere to the skin under adverse environmental conditions, there are certain situations in which any sunscreen should be reapplied at a maximum of 3- to 4-hour intervals:
- Continuous sun exposure, particularly between the hours of 10 A.M. and 3 P.M.
- Exposure at altitude of 7,000 ft (2,135 m) or higher
- Exposure within 20 degrees latitude of the equator
- Exposure during May through July in the Northern Hemisphere, and December through February in the Southern hemisphere
- Frequent water immersion, particularly with toweling off
- Preexisting sunburn or skin irritation
- Ingestion of drugs, such as certain antibiotics, that can cause photosensitization
Some authorities recommend using sunscreens of at least SPF 29, with the rationale that most people underapply or improperly apply them. Bald-headed men should protect their domes. All children should be adequately protected. However, avoid PABA-containing products in children less than 6 months old.
For total protection against ultraviolet and visible light, a preparation can be composed from various mixtures of titanium dioxide, red petrolatum, talc, zinc oxide, kaolin, red ferric oxide (calamine), and icthammol. These preparations or similar commercial products (“glacier cream”) are used for lip and nose protection. Sunscreens that prevent infrared transmission may help prevent flares of fever blisters caused by herpes virus.
Potential Complications from Sunscreen Use
If you are concerned about jellyfish stings, a useful product is Safe Sea Sunblock with Jellyfish Sting Protective Lotion, which is both a sunscreen and jellyfish sting inhibitor.
Substances that are ineffective as sunscreens and that may increase the propensity to burn include baby oil, cocoa butter, and mineral oil.
Taking aspirin or a nonsteroidal anti-inflammatory drug (such as ibuprofen) at 6-hour intervals three times prior to sun exposure may help protect the sun-sensitive person.
Many effective sunscreens, particularly those advertised to stay on in the water, are extremely irritating to the eyes, so take care when applying these to the forehead and nose. Near the eyes, avoid sunscreens with an alcohol or propylene glycol base. Instead, use a sunscreen cream.
Clothes to Protect You From the Sun
A line of medical clothing, Solumbra by Sun Precautions, is advertised to be “soft, lightweight and comfortable,” and offers 30-plus SPF protection. Solar Protective Factory also manufactures high-SPF protective clothing. Sunday Afternoons manufactures comfortable broad-brimmed hats with neck shields advertised to provide 97% UV block. Women’s hosiery has an unacceptably low SPF. The ability of Lycra to block UVR varies depending on whether it is lax (very effective) to stretched (nearly ineffective). Dry, white cotton (T-shirt) has an SPF of 5 to 8. The ultraviolet protection factor (UPF) is a measure of UVR protection provided by a fabric. Thus, a UPF of 15 indicates that 1/15 of the UVR that strikes the surface of the fabric penetrates through to the skin. A chemical UVR protectant, Tinosorb FD (Rit Sun Guard ), may be used as a laundry additive, increasing the UPF of washed clothing up to 50.
UVR protection provided by hats depends upon the style. Broad-brimmed hats and “bucket” hats provide the most protection for the face and head. Legionnaires hats do a decent job of protection, but baseball caps leave many facial areas exposed. If you are wearing a helmet, add a visor.
photo courtesy of www.school.discoveryeducation.com
Recent Blog Posts
Jul 01, 2013
In Advance of a Wildfire
Feb 11, 2013
Topical Ivermectin Lotion for Treating Head Lice
Feb 04, 2013
Public Health Interventions and Snowmobile Fatality Rates