Broad Spectrum
Authority: US FDA 21 CFR 201.327
In the US, a sunscreen can only be labeled 'Broad Spectrum' if it passes the FDA's critical wavelength test: measured on an in vitro spectrophotometer, the product must show a mean critical wavelength of at least 370 nanometers, meaning it absorbs UV radiation across the UVA range as well as UVB, not just UVB alone.
| Parameter / Specification | Standard Value | Unit |
|---|---|---|
| Minimum Critical Wavelength to Pass | 370 | nm |
| Test Method | In vitro spectrophotometer measurement on roughened PMMA plates | sunscreen applied at 0.75 mg/cm^2 |
| Critical Wavelength Calculation | Wavelength at which cumulative UV absorbance reaches 90% of total absorbance across 290-400 nm | - |
The FDA's Critical Wavelength Test
A sunscreen's SPF number reflects only UVB protection. To make a 'Broad Spectrum' claim in the US, a product must additionally pass a separate test defined in FDA regulation 21 CFR 201.327: an in vitro measurement using a spectrophotometer, with the sunscreen applied at a standard rate of 0.75 mg per square centimeter onto roughened PMMA (acrylic) plates that mimic skin texture. From the resulting absorbance curve, the lab calculates the 'critical wavelength' -- the point on the spectrum, moving up from 290 nm through the UV range, at which the cumulative absorbance reaches 90% of the total absorbance measured across the full 290-400 nm UV band.
A product qualifies as Broad Spectrum only if that critical wavelength is 370 nm or greater. Practically, this means the product's absorbance has to extend well into the UVA range, not just block UVB -- a sunscreen that filters UVB heavily but does little in the UVA range will show a lower critical wavelength and fail the test.
Why UVA Coverage Needed Its Own Rule
Before this rule took effect, US sunscreen labels could advertise UVA protection with no standardized way to verify the claim, and SPF numbers -- which measure only UVB defense -- gave consumers no information about it at all. UVA rays penetrate deeper into skin than UVB, contribute more to premature aging, and play a role in skin cancer risk, so a high-SPF product with weak UVA filtering could leave meaningful gaps in protection despite a reassuring label number.
The FDA finalized this testing requirement in its 2011 sunscreen labeling rule (76 FR 35660, effective June 17, 2011), giving 'Broad Spectrum' a specific, testable meaning for the first time in the US market. Only products that pass the critical wavelength test -- and that also meet SPF-proportional-to-UVA-protection criteria -- may use the term on their label; all others must either omit any UVA claim or, below SPF 15, carry a warning that the product doesn't protect against skin cancer or early aging.