Sunscreens

What is sunscreen?

Sunscreens are topical agents that can be applied to the skin to protect against ultraviolet (UV) radiation. 

Exposure to UV radiation (UVR) contributes to skin ageing and is the main cause of skin cancer (eg, basal cell carcinoma, squamous cell carcinoma, and melanoma). Sun protection methods, including sunscreen and sun protective clothing, are important in reducing these damaging effects.

The term ‘sunblock’, previously synonymous for sunscreen, is now considered misleading given that no sunscreen ‘blocks’ 100% of UVR. 

Use before going out and every 2 hours, and of sufficient thickness to maintain the high SPF

Develop the habit early on!

Modern sunscreen have micronised mineral contents and chemical absorbers that are less apparent

What is sunscreen used for?

Ultraviolet (UV) radiation can be classified as ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). Solar UVC (wavelength 100–280 nm) is absorbed by the Earth’s atmosphere. 

Sunscreen is used to protect the skin against the harmful effects of UVA and UVB radiation that reach the earth’s surface (see below for Benefits of sunscreen). It is recommended to apply sunscreen daily when the UV index is forecast to reach 3 or higher.

Table 1. Types of ultraviolet radiation

How do sunscreens work?

Chemical sunscreens work by absorbing UVR. 

‘Physical’ sunscreens (also called inorganic filters, ‘blockers’, or mineral-based sunscreens) were previously thought to reflect or scatter UVR. Studies have since shown that, like ‘chemical’ sunscreens, they provide UV protection primarily via absorption of UVR. 

Types of sunscreens

Chemical sunscreens

• Chemical sunscreens (organic filters or ‘absorbers’) often contain a combination of ingredients to provide both UVB and UVA protection.
• Types of agents include:
  • Aminobenzoic acid derivatives (eg, Padimate O)
  • Benzophenones (eg, oxybenzone)
  • Cinnamates (eg, octocrylene)
  • Salicylates (eg, homosalate).
• Photostable agents octocrylene, bemotrizinol, and bisoctrizole are often combined with other chemical absorbers to enhance the overall stability of the sunscreen.

Mineral/‘physical’ sunscreens

The most common physical sunscreens contain inorganic metal oxides, titanium dioxide, or zinc oxide, which:

• Reflect photons in visible light range, and work as chemical filters in the UV range
  • Titanium dioxide absorbs up to 400 nm
  • Zinc oxide absorbs up to 370 nm.
• Are chemically inert, safe, and provide broad spectrum UVA and UVB coverage
• Are photostable and often combined with photo-unstable ‘chemical’ agents.

Microsized or ultrafine grades (‘nanoparticles’) of titanium dioxide and zinc oxide have improved ease of application and cosmesis, and reduced white residue on the skin. Potential toxic or carcinogenic effects of nanoparticles have been raised, however, no adverse effects have been demonstrated. The established benefits of sun protection are considered to outweigh these theoretical risks.

Primary and secondary sunscreens

Sunscreens can also be considered either primary or secondary. In primary sunscreens, sun protection is the main function; while in secondary sunscreens (eg, makeup) it is not.

Sunscreen ingredients

Table 2 provides information on different sunscreen ingredients (non-exhaustive) and the protection they offer against UVA and UVB radiation. Many sunscreen products contain combinations of ingredients to optimise UV protection and photostability.

Table 2. Sunscreen agents and their coverage against UVA and UVB radiation

Note: recent proposed sunscreen regulation changes by the FDA (2019 and 2021) affect some of the above ingredients. Specifically:

• Titanium dioxide and zinc oxide are proposed as safe and effective.
• PABA and trolamine salicylate are proposed as not safe and effective.
• More information is needed to determine the safety of 12 other ingredients: padimate O, dixoybenzone, oxybenzone, sulisobenzone, octocrylene, octinoxate, cinoxate, homosalate, octisalate, avobenzone, ensulizole, and meradimate.

Sunscreen and the coral reef

Some chemical sunscreen ingredients have been banned in certain locations (eg, Hawaii, the US Virgin Islands, and Palau), due to concerns about the negative effects on coral reefs. These include:

• Oxybenzone
• Octocrylene
• Octinoxate
• Avobenzone.

In these locations, mineral sunscreens (zinc oxide and titanium dioxide) are generally preferred. Check local regulations for specific details and the most up-to-date information. 

Sunscreen protection ratings

Sunscreen ratings include information about protection against UVA and UVB protection. Sun protection factor (SPF) ratings pertain primarily to UVB protection as UVB is the primary cause of sunburn.

UVB protection rating

Sun protection factor (SPF) is the ratio of the UV radiation dose it takes to cause a barely detectable sunburn on a person treated with a sunscreen product (coverage of 2 mg/cm²) compared to that required for untreated skin.

While the length of exposure that results in sunburn varies between individuals, a way to conceptualise what the SPF number on a sunscreen product means is to think of it in terms of ‘difference in time to sunburn’. For example, if it takes 10 minutes to burn without sunscreen, skin protected with SPF 15 sunscreen (sufficiently applied) would be expected to take ~15 times as long to receive the same sun-burning UV radiation dose (ie, 150 minutes).

• The higher the SPF number, the better the expected protection.
• An SPF 15 sunscreen provides about 94% protection against UVB.
• Protection against UVB is increased to 97% with SPF 30 and 98% with SPF 50+.

Table 3. SPF ratings and protection against sunburn over duration of sun exposure

*Assumes application of 2 mg/cm². In reality, many people do not apply this amount of sunscreen so the expected sun protection may not be achieved.

UVA protection rating

More evidence about UVA-induced skin damage prompted a focus on methods to determine UVA protection, often referred to as a sunscreen’s ‘broad spectrum’ performance.

While there is no universally agreed standard for testing and measuring UVA protection, most countries have moved away from the in vivo persistent pigment darkening (PPD) method and now use in vitro methods. 

Measures include UVA protection factor (UVA-PF) and critical wavelength (CW). 

• UVA-PF: protection performance of a sunscreen against UVA radiation. Often considered as a ratio against the SPF value; many regulatory bodies require that UVA-PF is at least 1/3 of the SPF to be considered ‘broad spectrum’.
• CW: wavelength at which 90% of the cumulative area under the total solar radiation absorbance curve from 290–400 nm occurs. ‘Broad spectrum’ generally designates CW ≥370 nm, indicating greater protection against longer wavelength UVA rays.
• Some countries simply use a star rating to approximate UVA performance — 5 stars offering higher UVA protection.

Sunscreen testing

Due to ethical concerns about in vivo testing, in its 2006 Recommendation 2006/647/EC the European Commission encouraged the sunscreen industry to develop in vitro testing methods for both UVB and UVA protection.

UVB (SPF) testing

Currently, SPF testing is still done using in vivo methods, such as the international standard ISO 24444. 

Several in vitro testing methods are in development; the in vitro ‘double plate’ method (ISO Committee Draft 23675) and the hybrid diffuse reflectance spectroscopy method (ISO Committee Draft 23698) are in the final stages of international standardisation. These should become the first published ISO standards for in vitro SPF testing in the next few years. 

UVA testing

Table 4. UVA protection testing

*UVA-PF: ultraviolet A protection factor; CW: critical wavelength

Water resistance testing

Methods for testing sunscreen water resistance vary internationally, and include:

• FDA method — OTC Monograph M020.80(g) (Final Administrative Order, Sept 2021)
• COLIPA 2005 method (Guidelines for Evaluating Sun Product Water Resistance)
• International standards ISO 16217 (Water immersion procedure for determining water resistance) and ISO 18861 (Percentage of water resistance)

Testing currently involves in vivo protocols, as SPF is measured after immersion for the specified time period.

Sunscreen labelling

UVB and UVA ratings provide information for sunscreen labelling, so consumers can identify which products offer the best protection. Although category and labelling descriptions vary, Australia/New Zealand, Europe, the United States, and Canada are closely aligned. 

The terms ‘sunblock’, ‘waterproof’, or ‘sweatproof’ are considered inaccurate and misleading and no longer permitted on sunscreen labels.

Specific labelling guidelines are provided in Table 5 below. Cited guidelines may be updated or replaced; checking with the relevant body for the most up-to-date information is advised.

Table 5. Sunscreen labelling guidelines

Category descriptions to indicate the overall degree of sun protection conferred (low, medium, high, or very high) are also recommended for sunscreen products in Australia/New Zealand and the EU. Table 6 provides details on what these categories mean.

Table 6. Sunscreen protection categories*

*As per AS/NZS 2604:2021 and European Commission Recommendation (2006/647/EC)

**Select secondary sunscreen products (lipsticks and colour cosmetic products eg, tinted bases or foundations) are exempt from the UVA requirement as per AS/NZS 2604:2021

Sunscreen selection

General recommendations for selecting an appropriate sunscreen based on particular characteristics and conditions are outlined below in Table 7.

Table 7. Sunscreen selection advice

Sunscreen application

• Check sunscreen is within the use-by date.
• Apply daily if the ultraviolet index is 3 or higher.
• Expected sun protection is only provided if sunscreens are applied in quantities similar to the ones used for testing (2 mg/cm²). This equates to about 35mL of lotion (approx. 7 teaspoons) for an average-sized adult — requirements may vary depending on body habitus and clothing:
  • ~4 teaspoons for the limbs (1 for each arm and each leg)
  • 1 teaspoon each for the front and back of the trunk/torso
  • 1 teaspoon for the head (face, ears, and neck).
• Should be applied at least 20–30 minutes before sun exposure.
• Reapply every 2 hours in the setting of ongoing sun exposure.
• Reapply after swimming, heavy sweating, or if partially rubbed off (eg, by towel use).
• Sunscreen should be used in conjunction with other aspects of sun protection including sun protective clothing, avoiding sun exposure during peak sunshine hours (approx. 10am to 4pm in summer), and staying in the shade where possible.
• Insect repellents may reduce the SPF of sunscreen so when using both together, apply a sunscreen with a higher SPF and re-apply it more often.
  • In 2019, the FDA proposed designating combination sunscreen and insect repellent products as not safe or effective. 

What are the benefits of sunscreen?

Sunscreen is particularly useful for those with fair skin such as skin phototypes I, II, and III. 

The benefits of wearing a broad-spectrum sunscreen include:

• Avoidance of sunburn
• Reduction in risk of skin cancer
• Reduced hyperpigmentation
• Reduced photoageing, for example:
  • Wrinkles
  • Solar lentigines
  • Actinic keratoses
  • Loss of elasticity
• Avoidance/reduced aggravation of photosensitive dermatoses. 

What are the disadvantages of sunscreen?

• Many people do not apply sufficient sunscreen to optimise sun protection.
• No sunscreen blocks 100% of ultraviolet radiation or is fully waterproof or sweatproof.
• Some sunscreen formulations or products may be less preferable depending on skin type or cosmetic appearance. Fortunately, there are a range of sunscreens available.
• Some chemical sunscreens may impair coral reef growth and contribute to coral bleaching.
  
  • Scientific regulation around sunscreen products marked as ‘reef-safe’ is an evolving area.
• Many sunscreens that protect against UVR do not protect against visible light. Those with photodermatoses triggered or aggravated by visible light (eg, melasma, postinflammatory hyperpigmentation, solar urticaria, cutaneous porphyrias, and some cases of chronic actinic dermatitis) should consider using tinted sunscreens (eg, ‘Dundee cream, available in 3 different colours’) which contain non-micronised titanium dioxide, zinc oxide, and iron salts, as only sunscreens that remain visible on the skin will photoprotect against visible light.
• Sunscreens have potential side effects (see below).

What are the side effects and risks of sunscreen?

Common side effects include:

• Transient stinging following application
• Skin irritation
• Rash (usually an irritant contact dermatitis).

Less commonly, allergic contact dermatitis may develop due to a sunscreen component, eg, a fragrance, preservative, or a sunscreen chemical.

Sunscreen allergy has been reported to individual sunscreen agents including PABA, benzophenones, cinnamates, salicylates, avobenzone, ecamsule, ensulizole, bemotrizinol, and bisoctrizole. This may be either an allergic or a photocontact reaction.

The FDA has proposed that PABA (aminobenzoic acid) lose its status as GRASE (generally recognised as safe and effective) due to safety concerns including significant rates of allergic contact dermatitis and photocontact dermatitis, and cross-sensitisation that may cause allergies to other medications (eg, sulfonamide antibiotics, thiazide diuretics, and some local anaesthetics). PABA use in sunscreen products is now less common.

Other sunscreen safety considerations

Systemic absorption of chemical sunscreens

• Studies have demonstrated systemic absorption following topical use of a number of chemical sunscreen agents including avobenzone, oxybenzone, octocrylene, ecamsule, homosalate, octisalate, and octinoxate (Matta et al, 2020).
• Animal and in vitro studies have also demonstrated that some of these agents (eg, homosalate, oxybenzone, and octinoxate) have the potential to affect endocrine activity (Krause et al, 2012; Lorigo et al, 2018).

Research into the clinical significance of these findings is ongoing. The FDA has proposed that further information is needed prior to regarding these and other sunscreen agents (except zinc oxide and titanium dioxide) as GRASE (generally recognised as safe and effective). However, this does not conclude that chemical sunscreens are unsafe for use, and sun protection should remain a priority.

Trolamine salicylate has been associated with cases of systemic absorption and adverse reactions including increased bleeding risk and salicylate toxicity. It is no longer commonly used in sunscreen products and has been proposed to lose its GRASE status.

Nanoparticles

Safety queries relating to zinc oxide and titanium dioxide ‘nanoparticles’ have been raised regarding reactive oxygen species, the potential for systemic absorption, and possible associated carcinogenicity. Research in this area is ongoing.

To date, no adverse effects have been demonstrated and the general consensus is that the well-established benefits of sun protection outweigh any potential long-term risks. 

Vitamin D

• Concerns that sunscreen protection against UVB radiation, which is involved in Vitamin D production, could contribute to Vitamin D deficiency, were not shown with real-life recreational sunscreen use in randomised controlled trials.
• Vitamin D supplements (oral capsules) are available if deficiency is a concern.

For more information, see Vitamin D.

What are the contraindications to sunscreen?

• Hypersensitivity reactions to an ingredient (see: Sunscreen allergy).
• Some sunscreen ingredients are banned in certain locations (eg, Hawaii, Palau, and the US Virgin Islands) due to concerns about coral reef protection; check local guidelines before travelling.