page_title: UV filter
section_title: P-aminobenzoic acid (PABA)
hierarchical_section_title: UV filter / Fate of some Organic UV filters / P-aminobenzoic acid (PABA)
caption_attribution_description: English: Photolysis of p-aminobenzoic acid in water
context_page_description: UV filters are compounds, mixtures, or materials that block or absorb ultraviolet light. One of the major applications of UV filters is their use as sunscreens to protect skin from sunburn and other sun/UV related damage. After the invention of digital cameras changed the field of photography, UV filters have been used to coat glass discs fitted to camera lenses to protect hardware that is sensitive to UV light.
context_section_description: P-amino benzoic acid was one of the earliest UV filters used in sunscreens (1943). It was used in concentrations up to 5%. It was discovered by 1982 that PABA increases the formation of a particular DNA defect in human cells. The photochemical fate of PABA may be impacted by water constituents, e.g., NO3⁻, dissolved organic matter (DOM), and HCO3⁻. PABA undergoes both direct and indirect photolysis in the solution with the presence of NO₃. Direct photolysis accounts for 25% of the degradation of PABA and is considered a secondary pathway. On the other hand, indirect photolysis was the dominant pathway.
  Zhou and Mopper showed that nitrate enhanced the photodegradation of PABA by a factor of 2. However, in the presence of free radical scavengers such as carbonate forms and natural organic matter (NOM), the photodegradation of PABA decreased. It was proposed that the indirect photolysis of PABA was mainly due to the NO₃ photolysis product •OH.
  The Bicarbonate anion is abundant in water. Bicarbonate caused 10% of •OH scavenging. The reaction between bicarbonate and the •OH yields carbonate radical (•CO3), which is less reactive than •OH. In natural waters, •CO3 can reach a higher steady-state concentration than •OH because of its lower reactivity. The enhancement of PABA photolysis by bicarbonate is due to carbonate radicals.
  Water-soluble NOM is composed of organic acids. These organic acids are mainly humic substances, which can be categorized into a fulvic and humic acid fraction. NOM favors the indirect photolysis of PABA by absorbing the sunlight and weakening its intensity.
  Two reactions can take place during the degradation of PABA in the presence of nitrate in water as shown in the figure. Three of the four products contain phenolic groups and may thus be estrogenic. So the hazardous byproducts generated during the PABA photoreaction should be concerned for its estrogenicity.
caption: Indirect photolysis of p-aminobenzoic acid UV filter in water
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