How Is SPF Tested? Step-by-Step Methods Used in Labs & At Home

SPF, or Sun Protection Factor, refers to the level of a sunscreen's efficiency in protecting the skin from UVB rays - short for the ultraviolet radiation that causes sunburn and skin cancer. However, did you ever ask yourself how is SPF tested before the number comes into view on your sunscreen packaging?

For brands, understanding SPF testing is crucial for regulatory compliance and consumer loyalty. In this article, we will dig into how is SPF tested, not only through the professional laboratory method but also through the simplified at-home method. Moreover, we will consider how the stages of SPF testing are in accordance with global quality and safety standards for chemicals like Restricted Substance List (RSL) testing, CA Prop 65, and EU REACH compliance.

Accurate SPF testing for brands and formulators is a matter of first importance - not only for efficacy claims but also for chemical risk management and making sure that products do not contain hazardous substances.

What Does SPF Really Measure?

SPF is a way to quantify how much longer the skin covered with a sunscreen of a given protection factor can tolerate the exposure to UVB rays before an erythema (sunburn) induced by UVB appears, compared to unprotected skin.

For instance, your skin is assumed to burn in 10 minutes. Hence, using SPF 30 should, in theory, prolong that time to 300 minutes.

SPF is mainly focused on the UVB side of the spectrum; however, a product's UVA protection is also evidenced by further tests, for instance, in vitro photostability.

How Is SPF Tested in Laboratories?

Step 1: Ethical Human Panel Selection

Test of a professional solar protection factor, for instance, the one made by Eurofins CRL Photobiology Services, is a human-based experiment with volunteers whose skin types are between I and III (very fair to light brown).

To identify the Minimal Erythema Dose (MED) - the minimum UV energy that provokes redness after 24 hours, each tested individual’s back or arm is irradiated with a graded UV exposure.

Step 2: Application of Test Sunscreen

Typically, a sample of the sunscreen is given a dosage with a controlled amount, generally 2 mg/cm², on the designated test areas for testing. Then the UV exposure is carried out after about 15 minutes of waiting for the product to be absorbed.

It is crucial to use this measured amount for the results to be comparable and also in line with different standards such as ISO 24444:2019 and FDA 21 CFR 352.

Step 3: Controlled UV Exposure

Step-by-step, they give each test site higher and higher UV doses using a solar simulator. Such simulators produce a controlled UVB radiation that matches natural sunlight quite well. 

They then check the test areas after exposure to find MED values for those parts of the skin that have been covered as well as those that have not. 

Step 4: Calculating SPF Value

The SPF number is calculated using the formula:

SPF = MED (Protected Skin) ÷ MED (Unprotected Skin)

If the protected skin requires 30 times more UV exposure to redden than unprotected skin, the SPF rating is 30.

Step 5: Validation and Reporting

After all readings have been collected, the results are averaged across volunteers. The final SPF value is given with a statistical confirmation.

Testing documents from the lab - such as those generated by Eurofins CRL Photobiology Labs - also specify the procedure, the type of instrument used for calibration, and standards used as references.

Such reports indicate that the product complies with the international standards in photobiology for promotional claims and regulatory filings.

In Vitro SPF Testing: Non-Human Alternative

Testing of SPF by in vitro means faster and more humane methods, as well as an excellent alternative. Such a test involves the measurement of UV transmittance across a very thin layer of sunscreen placed on a base (e.g., PMMA plates).

Steps:

• Sample Preparation – The sunscreen is spread out evenly on the plate to form a uniform film.
• Spectrophotometric Analysis – The UV absorption of the sample is taken over the range of wavelengths.
• Data Interpretation – Computational methods provide estimates of the SPF that can be compared to those from human tests.

Even though in vitro testing ensures standardization and reproducibility, it is frequently used as a backup to in vivo (human) testing for regulatory labeling.

How to Test SPF at Home (Simplified Methods)

Home SPF testing has its limitations; it is unable to achieve lab accuracy. However, both consumers and formulators can conduct some preliminary assessments to get an idea of the product.

Method 1: UV Detection Stickers or Beads

UV-changing stickers are quite simple in principle: the color changes depending on how much UV they get. Hence, if sunscreen is applied only on one sticker while the other is left untreated, users can, to a certain extent, figure out the relative protection by observing the color difference.

Method 2: Controlled Sunlight Exposure Test

A simple DIY method for comparison is to put sunscreen on one part of your skin and leave the other one unprotected. After the same amount of sun exposure, check for the appearance of redness in both areas. (Warning: Do not expose yourself to the sun for a long time and always make use of a proper sun protection method.)

Method 3: Spectrophotometer Devices

Small UV meters and spectrophotometers for the field can roughly estimate SPF values through light absorption. However, these have to be correctly set up and operated by someone trained to create accurate results.

Why Reliable SPF Testing Matters

Accurate SPF testing ensures consumer safety and compliance with:

• FDA regulations (U.S.)
• ISO and COLIPA standards (EU)
• Therapeutic Goods Administration (TGA, Australia)

Beyond UV efficacy, companies must ensure sunscreens are chemically safe and free of restricted substances. This is where RSL testing and chemical risk assessment come in.

Connection Between SPF Testing & RSL (Restricted Substance List) Compliance

RSL and SPF testing share the same endpoint objective, which is consumer protection. Besides that, SPF checks the sun-block safety, and RSL works to ensure that there are no harmful chemicals in the product or the packaging.

What Is RSL Testing?

The purpose of RSL (Restricted Substance List) testing is to locate and measure the hazardous chemicals that can bring harm to health and/or the environment. It is a leading tool for use in the apparel industry in the area of garment and apparel RSL testing, as well as in cosmetic formulation control and chemical risk management fields.

Common RSL Testing Parameters:

• Heavy metals (Lead, Cadmium, Mercury)
• Phthalates & BPA (covered under CA Prop 65)
• Aromatic amines and formaldehyde
• PAHs and allergens

Understanding what is RSL testing helps sunscreen brands ensure that formulations comply with global safety standards, like:

• Annex XVII of the EU REACH Regulation
• CA Prop 65 compliance testing
• AFIRM RSL 2025 updates

When combined with SPF efficacy testing, RSL and chemical risk assessments ensure sunscreens are both effective and safe for consumer use.

Role of Eurofins CRL Photobiology Services

Eurofins CRL Photobiology is a global leader in SPF and photobiology testing. Their laboratories offer:

• In vivo SPF testing (ISO 24444, FDA, COLIPA)
• In vitro UVA and photostability analysis
• Water-resistance and sweat-resistance evaluations
• Customized RSL and chemical safety assessments

Through the use of leading solar simulators and the expertise of their scientists, Eurofins can guarantee your sunscreen meets global SPF validation standards, as well as ensuring that the product is kept chemically safe via RSL and CA Prop 65 procedures.

Regulatory Compliance and Risk Management

A sunscreen product can only hit the market if it satisfies two essential pillars:

• Photobiological performance - a Proven SPF rating.
• Chemical safety - results from RSL testing, Prop 65, and REACH compliance.

An integrated chemical risk management plan outlines:

• Chemical supply chain risk evaluations
• High-risk chemical testing
• RSL testing report maintenance for audits and certifications

This dual compliance model is useful in protecting companies against the risk of both human health and brand reputation.

Frequently Asked Questions

1. How is SPF tested in laboratories?
   
   Primarily, tanning experiments with human volunteers, which are controlled, are the main methods to test SPF in vivo. Upon the application of the very controlled UV radiation, the minimal erythema dose (MED) is recognized.
   
   
2. How to test the SPF of sunscreen at home
   
   One can employ UV stickers, spectrophotometers, or comparative sunlight exposure tests to get a rough comparison. However, the data given in these methods are unsuitable for labeling purposes.
   
   
3. What is the most accurate SPF testing method?
   
   The human in vivo test (ISO 24444) is the most accurate method for the SPF test and is the gold standard for the determination of SPF.
   
   
4. What is RSL testing, and why is it relevant to sunscreen?
   
   RSL testing refers to a method of identifying restricted chemicals in the ingredients of the sunscreen that are safe and comply with environmental safety regulations worldwide.
   
   
5. Why choose Eurofins CRL for SPF testing?
   
   Eurofins CRL is a globally accredited laboratory, and it provides total photobiology and chemical safety compliance with SPF, UVA, and RSL as per the requirements of different standards.

Conclusion

SPF testing is a scientific and regulatory measure. It has been a major challenge to combine the experimental in vivo human trials and the new in vitro technologies for a successful SPF testing method. Government agencies must keep up with the progress to be able to certify the safety and the legality of the labeled products through how SPF is tested.

Moreover, by choosing a laboratory with an international reputation, such as Eurofins CRL Photobiology, a manufacturer of sunscreens can not only make sure of the reliability of their product but also be sure of RSL and CA Prop 65 compliance and, therefore, reassure customers of a safe, efficient, and sustainable sun care product.