2026-07-10 · 3 min read
UV-A vs UV-B filters: how to build a broad-spectrum sunscreen
What the two UV bands do to skin, why a compliant SPF needs both covered, and how formulators combine oil-soluble, water-soluble and mineral filters to get there.
Sunlight that reaches skin carries two ultraviolet bands that matter to a formulator. UV-B (~290–320 nm) is the burning band: higher energy, absorbed in the epidermis, the driver of sunburn and the SPF number itself. UV-A (~320–400 nm) penetrates deeper into the dermis; it is the ageing band, responsible for photo-ageing and a large share of long-term skin damage, and it passes through window glass.
A sunscreen that only handles UV-B earns a high SPF label while leaving the skin exposed where damage is slowest to show. That is why every major market now expects broad-spectrum protection; in the EU, UV-A protection must be at least a third of the labelled SPF.
Covering the spectrum
No single organic filter covers the whole range well, so formulas combine:
- UV-B specialists: strong absorbers peaking around 305–315 nm that carry the SPF number efficiently at low dosage.
- UV-A specialists: filters with absorption maxima deep in the UV-A band that protect where the burn test doesn't look.
- Broad-spectrum bridges: modern triazine filters that absorb across both bands and are highly photostable, so the protection you measure at t=0 is still there hours into wear.
- Mineral filters: zinc oxide and titanium dioxide, which scatter and absorb across the spectrum and anchor naturals-positioned and sensitive-skin formulas.
Here are the measured bands for the Sunbest filters, read straight from their datasheets and plotted on the 280–400 nm axis. A formulator reads these the way a sound engineer reads frequency response: the goal is flat, even coverage with no gaps.
Three practical rules
- Photostability first. A filter that degrades in sunlight takes its protection with it. Check retained absorbance after irradiation, not just the fresh extinction curve.
- Mind the phases. Oil-soluble filters live in the emulsion's oil phase and drive richness; water-soluble filters let you build SPF without greasiness. The split shapes the sensory profile as much as the emulsifier choice does.
- Boost before you overload. SPF boosters raise measured protection by improving film formation, often a better route to the last few SPF points than adding more filter.
Vinner Labs' Sunbest line spans all four groups: UV-B and UV-A specialists, broad-spectrum triazines, minerals and boosters, each with absorption-band data on its datasheet. Browse the Sun Care ingredients or ask us for a starting formulation.
Frequently asked
Why does a sunscreen need more than one UV filter?
Because absorbance curves are narrow. UV-B specialists peak around 305–315 nm, UV-A specialists sit deep in the 320–400 nm band, and only a few triazines bridge both. Combining them is what produces even cover with no gaps.
What does the EU require for a broad-spectrum claim?
A UV-A protection factor of at least one third of the labelled SPF, and a critical wavelength of at least 370 nm. Products meeting it carry the UVA-in-a-circle mark.
Should I use oil-soluble or water-soluble filters?
Both, usually. Oil-soluble filters sit in the emulsion's oil phase and drive richness; water-soluble filters let you build SPF without greasiness. The split shapes sensory profile as much as emulsifier choice does.