What 'Broad Spectrum' Means and How to Tell If a Sunscreen Earns It

Broad spectrum is an FDA-regulated claim that requires a sunscreen to achieve a critical wavelength of 370nm or greater, meaning its protection extends meaningfully into the UVA range, not just UVB. Swellies was independently lab-tested and came back with a critical wavelength of 381.67nm and a UVA I/UV ratio of 0.93. Most brands meet the minimum threshold and never publish the actual numbers. Here is what those numbers mean and why they matter.

What Does "Broad Spectrum" Mean?

Broad spectrum is an FDA-regulated claim. To use it, a sunscreen must achieve a critical wavelength of 370nm or greater, meaning its UV-absorbing ability extends deep enough into the UVA range to qualify as protective across the full UV spectrum, not just UVB.

UVB rays are the ones responsible for sunburn. UVA rays penetrate deeper into skin and are present year-round, even on overcast days. A sunscreen that only blocks UVB is doing half the job, which is why broad spectrum coverage matters.

The FDA requires a critical wavelength of 370nm or greater for a broad spectrum claim. A higher number means protection extends further into the UVA range.

How Is Broad Spectrum Tested?

The test is an in vitro (lab-based) procedure using UV spectrometry. The sunscreen is applied to a PMMA plate, irradiated to simulate sun exposure, and then measured across the full UV spectrum from 290 to 400nm. The critical wavelength is the point at which the cumulative absorbance of the formula reaches 90% of the total UV spectrum. If that point lands at or above 370nm, the product passes.

It is a pass/fail threshold, but the score matters. A product that scrapes by at 371nm and one that hits 381nm both get the "broad spectrum" label. They are not the same.

Swellies' Numbers, Straight from the Lab

Swellies was tested by an independent FDA-compliant sunscreen testing lab in February 2026. The results show a critical wavelength of 381.67nm and a UVA I/UV ratio of 0.93.

That 381.67nm critical wavelength is not just a pass. It is nearly 12nm above the minimum threshold. The UVA I/UV ratio of 0.93 reflects how much of the formula's total UV protection falls in the UVA I range (340 to 400nm), the wavelengths most associated with deeper skin penetration. A ratio closer to 1.0 means more of the protection sits in that harder-to-cover UVA range.

Why Non-Nano Zinc Oxide Performs This Way

Zinc oxide is one of only two UV filters approved by the FDA that provides genuine broad spectrum coverage on its own. It absorbs and reflects UV radiation across both UVB (290 to 320nm) and UVA (320 to 400nm), which is why a formula built around 21% non-nano zinc oxide can achieve the kind of UVA coverage reflected in Swellies' test data.

Chemical sunscreens typically use multiple UV filters because individual chemical actives are each effective only within narrow wavelength bands. That is why you will often see five or six actives on a chemical sunscreen label, they are covering different slices of the spectrum in combination. Zinc oxide covers it alone.

Swellies uses 21% non-nano zinc oxide as its only UV filter. No hidden actives. No butyloctyl salicylate. No boosters. Just zinc.

Does Mineral Sunscreen Always Leave a White Cast?

This is the question everyone has about zinc oxide, and it is fair because most mineral sunscreens do. The white cast comes from zinc oxide particles sitting on top of skin rather than integrating with it. Swellies uses polyhydroxystearic acid, a castor oil-derived ingredient, specifically to prevent that. It coats the zinc oxide particles and allows them to disperse evenly so the formula goes on sheer. The iron oxides add a subtle tint that neutralizes any remaining cast on skin and on facial hair.

Why Don't More Brands Publish Their Test Data?

They are not required to. The FDA requires that broad spectrum testing be done and kept on file, but there is no mandate to make it public. Most brands do not. We think that is worth changing, even in a small way, which is why Swellies' test results are here for anyone to read.

Five ingredients. Third-party tested. Numbers on the table.

Frequently Asked Questions

What sunscreen has the best broad spectrum coverage?

Among single-active mineral sunscreens, Swellies tests at a critical wavelength of 381.67nm with a UVA I/UV ratio of 0.93, well above the 370nm minimum required for an FDA broad spectrum claim. Most brands meet the threshold and stop. Swellies publishes the actual numbers.

What does broad spectrum mean on sunscreen?

Broad spectrum is an FDA-regulated term. To earn it, a sunscreen must pass the Critical Wavelength Test, achieving a critical wavelength of 370nm or greater. This confirms the formula absorbs UV across both the UVB range (280–315nm) and meaningfully into the UVA range (315–400nm). Swellies tests at 381.67nm.

What is the difference between UVA and UVB protection?

UVB causes sunburn. UVA penetrates deeper and causes cumulative damage. SPF measures UVB protection only. Broad spectrum testing measures UVA protection. You need both. Zinc oxide, the only active in Swellies, covers the full UV spectrum alone at 21%.

What is a critical wavelength in sunscreen testing?

Critical wavelength is the point at which a sunscreen has absorbed 90% of UV energy across the spectrum. The FDA requires 370nm as the minimum for a broad spectrum claim. A higher critical wavelength indicates broader, more even UVA coverage. Swellies comes in at 381.67nm, nearly 12nm above the threshold.

What is a UVA I/UV ratio?

This ratio measures how much of a sunscreen's total UV protection falls in the UVA I range (340–400nm), the deepest part of the UVA spectrum. A ratio closer to 1.0 means protection is well-distributed across the full UV range. Swellies scored 0.93. Most brands do not publish this number at all.

Why don't most sunscreen brands publish their broad spectrum test results?

The FDA requires the claim to be earned, but does not require brands to publish the actual test data. Most brands meet the minimum and move on. Swellies publishes the full numbers, critical wavelength and UVA I/UV ratio, because transparency is part of the product. Five ingredients, third-party tested, results on the table.