In-vitro vs in-vivo testing: How to measure the efficacy of your sunscreen

The dubiety associated with sunscreen isn’t new; the claims about SPF (Sun Protection Factor) pasted bold and clear on the packaging labels are often rigged or exaggerated. In the past, several brands have gotten away with doing so, given the lackadaisical rules and regulations pertaining to the legal SPF measurement standards in India. However, in November 2025, the Bureau of Indian Standards (BIS) was forced to revisit the traditional methods of SPF approvals, owing to the controversy sparked on social media.

While Lakmē’s ‘Sun Superiority’ campaign sparked a heated debate about the ethics and efficacy of sunscreens in India, New Delhi-based content creator Nitin Joshi’s sunscreen examination Reel on Instagram topped the virality charts, taking the debate forward. Until these controversies emerged, brands were permitted to use the SPF ratings procured via in-vitro testing, which refers to the test where sunscreen was placed on PMMA or Polymethyl Methacrylate, a transparent, rigid thermoplastic that was believed to mimic the properties of the skin. However, the BIS has now made it mandatory for every sunscreen to be tested under the in-vivo method, which refers to testing on real human skin. 

Headlined by increasing transparency and stringent guidelines, the new rules have now compelled beauty and skincare brands to rewire their testing approaches, instilling a renewed sense of assurance and reliability among consumers. 

But, why is in-vivo testing paramount? And, what really gives it an edge over its counterpart? We speak to dermatologists to decipher.  

Understanding what SPF really means

Every sunblock tube is imprinted with an SPF claim. ‘SPF’, which literally means ‘Sun Protection Factor’, measures the sunscreen protection against UVB rays, notes cosmetologist Dr Geetika Mittal Gupta. “SPF reflects how much, and for how long, can your skin be protected against the rays responsible for sunburn, and skin cancer.” Celebrity dermatologist Dr Madhuri Agarwal adds, “For example, SPF 15 means you can stay in the sun 15 times longer before the skin begins to burn. To determine SPF, researchers would typically apply a fixed amount of sunscreen to volunteers’ backs, then expose them to the UV light and measure how much UV exposure the sunscreen-protected skin can withstand before reddening as compared to unprotected skin.”

Can the SPF claims on labels be trusted?

SPF 15, 20, 30, 50, 75, and even 100—the number has turned into a sunscreen flex from a simple metric of protection for brands. The outcome? Consumers are more confused than confident. Sunscreen regulation and manufacturing standards can differ substantially across countries and companies, cites Agarwal.

“In India, the BIS’ guidelines set benchmarks for sunscreen labels; however, the enforcement of these guidelines are inconsistent. Some sunscreen brand manufacturers follow the comprehensive testing protocols strictly, whereas others may rely loosely only on ingredient calculations or minimal evaluation. Certain independent evaluations of sunscreens have demonstrated that there are discrepancies between a sunscreen’s labelled SPF and its actual protection level,” explains Agarwal. According to the medical professional, these discrepancies may not always be intentional—poor storage conditions and a formula’s questionable stability also count. “At times, the SPF could be 50 during the testing phase, but can drop to 35 due to improper storage.”

Besides SPF claims, application is also a key determinant of the sunscreen’s efficacy, says Mittal Gupta. “During testing processes, sunscreens are applied generously and evenly on the skin, and in conditions that are generally not very easy to replicate in real life. Most people apply a very small quantity as per recommendation; they skip the reapplication, and sometimes, rely only on SPF-infused make-up, which is not sufficient. Even though the SPF on the product’s label may be accurate, its efficacy in terms of protecting the skin falls short because most people are not following the general rules.”

In-vivo versus in-vitro testing: Identifying the best test to measure sunscreen’s SPF

In the wake of the recent debate around SPF, brands are now investing heavily into marketing and establishing their in-vivo testing standards to explain why it is important.

The term ‘in-vitro’ translates to ‘in glass’, referring to laboratory experiments performed outside of living organisms. On the other hand, ‘in-vivo’ means ‘in life’, denoting tests carried out directly on living skin. “While the former is a fast, cost-effective method and does not involve human subjects, the catch is that artificial skin does not replicate human skin precisely. It will lack pores, natural oils, sweat, hair follicles, and an uneven surface. It also does not move or crease,” explains Agarwal. “When sunscreen is applied to a real face, it will settle into pores, mix with sebum, and may not always spread evenly. In-vitro testing will not be able to fully account for these practical variables.” Which is why, for SPF measurement, in-vivo testing is the ideal standard required by most regulatory authorities including the Food and Drug Administration (FDA), the European Commission, and now even the BIS.

In-vitro testing isn’t entirely futile or gimmicky, but relying solely on it is what leaves room for discrepancies. “In-vitro tests help assess how UV filters behave—whether they absorb, scatter, or reflect UV light; and give early insights on formulation performance,” says Sapna Singh, research and development expert at Lakmē Skin. “In-vivo tests measure how long protected skin takes time to show erythema when compared to unprotected skin, which is the only globally accepted way to derive an accurate SPF value. Both methods matter; while in-vitro helps optimise the formula, in-vivo confirms the real-world efficacy on skin.” 

Expert-recommended do’s and don’ts of buying sunscreen

Getting sun protection right involves research, trust, and the right application of sunscreen. The first step to getting the right sunscreen is by verifying the labels—and not trusting them blindly. “Choose sunscreens that specifically mention testing under ISO standards (ISO 24444 for SPF, ISO 24442 for UVA protection). In India, check for BIS certification, if available. Global brands, on the other hand, often adhere to stricter guidelines because they must meet US FDA or EU regulations,” advises Agarwal. 

Next, try and experiment with a quick stability test on your own. “Sunscreen should have a consistent texture; if it separates into a watery liquid and thick paste or if it changes colour or smells odd, it is probably degraded and should not be used,” says Agarwal. “Beyond the label, efficacy shows up on your skin—it should experience less tanning, burning, and post-sun dullness with consistent use and proper reapplication,” adds Singh.

When choosing a sunscreen, look for an SPF of 30 or higher and a broad-spectrum PA rating, ideally PA+++ or PA++++. Choose a texture that suits your skin type, so you’ll use it consistently, notes Singh. Mittal Gupta concurs, “What makes a sunblock truly effective is its texture that allows you to apply and reapply in the right quantity. If you don’t like the texture and are not applying enough, then it is useless.”

Bookmark these sunscreen application tips

• Choose a sunscreen suited to your skin type and lifestyle. For example, individuals with oily or acne-prone skin should opt for gel-based or fluid formulations labelled as ‘non-comedogenic’. Those with dry skin can choose cream-based sunscreens containing added moisturisers.

• For activities involving swimming or excessive perspiration, water-resistant formulas are recommended. 

• It is important to apply an adequate amount of sunscreen. The recommended amount is approximately one teaspoon for the face and neck and sunscreen equivalent to a shot glass for the entire body.

• Reapplication every two hours while outdoors is essential.

• Don’t assume that a higher SPF automatically gives you significantly more protection. For instance, SPF 30 blocks around 97 per cent of UVB rays, SPF 50 offers about 98 per cent, and SPF 100 covers roughly 99 per cent. The increase from SPF 50 to SPF 100 is minimal; however, higher SPF sunscreens might be harsher on your skin due to greater amounts of UV filters. For everyday use in India, SPF 30–50 is usually sufficient as long as you apply it regularly and properly.

• Don’t skip often-overlooked areas like your ears, the nape of your neck, and hands.