What 'Broad Spectrum' Really Means (the 370 nm Test)
'Broad spectrum' isn't a marketing word — in the US it's a measurable FDA pass/fail line at 370 nanometers.
To be labeled 'Broad Spectrum' in the US, a sunscreen must pass an in-vitro test (21 CFR 201.327) proving a critical wavelength of at least 370 nm — the point at which 90% of its UV absorbance from 290-400 nm is reached.
The U.S. Food and Drug Administration defines 'Broad Spectrum' through a standardized test, meaning a product carrying the term has demonstrably met a measurable UVA-protection threshold rather than simply claiming it.
Questions we get a lot
What does 'broad spectrum' mean on a sunscreen?
It means the product protects against both UVA and UVB rays. In the US, the FDA only permits the term when a sunscreen passes a standardized in-vitro test demonstrating a critical wavelength of at least 370 nm. So a broad-spectrum label reflects a measured threshold, not just a marketing claim.
What is the 370 nm critical wavelength test?
Critical wavelength is the point at which 90% of a sunscreen's UV absorbance across 290-400 nm has been reached. Under FDA rules (21 CFR 201.327), a product must reach at least 370 nm to be labeled 'Broad Spectrum.' The higher the critical wavelength, the more of the UVA range the product covers.
r/SkincareAddiction: 'Is "broad spectrum" on a US label actually regulated or can any brand just slap it on?'