Figure 4.The effect of particle size on the UV attenuating properties of titanium dioxide.

Figure 4. The effect of particle size on the UV attenuating properties of titanium dioxide. Reduction of particle size moves the peak of UV attenuation to shorter wavelengths and also improves the transparency. However, the SPF efficacy is considerably reduced when the particle size is too small. The smallest particles (about 20 nm) do not exhibit real UV-protective benefits any longer, not even in the UVB, but such particles are still larger than the size of the quantum dots that were shown to penetrate pig26 and rat skin.27 (Modified from Reference 29.)