Toxic sunscreen testing

Simultaneous determination of an inorganic UV filter used in sunscreen, titanium dioxide, and several minor, trace or toxic, elements has been undertaken using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The multi-element analysis of sunscreen creams and lotions was compared with atomic absorption spectroscopic data.

George Zachariadis and E Sahanidou of the Laboratory of Analytical Chemistry, at Aristotle University, in Thessaloniki, Greece, offer details of the study in the October issue of the Journal of Pharmaceutical and Biomedical Analysis.

Sunscreen products are designed to protect one's skin from sunburn, reduce premature aging of the skin, and reduce apparent skin cancer risk. As such, they contain one of two categories of filters for ultraviolet light and sometimes both. The first, inorganic UV filters, reflect, scatter, or absorb a wide range of ultraviolet wavelengths. The second, organic UV filters, attenuate ultraviolet by absorbing the radiation. Different combinations of inorganic and organic filters can block UVA (320 to 400 nm) and UVB (290 to 320 nm) to varying degrees. None works perfectly to offer full protection against the detrimental effects of solar photochemistry on the skin, however.

Of course, there are regulations that limit the maximum amounts of both types of filter that are allowed in the manufacturing of sunscreens because there are toxicity risks associated with their use. The European Union and USA have lists of authorized UV filter compounds and allowable concentrations. On those lists is the bright-white compound, titanium dioxide, which is the only inorganic UV filter authorized for use in sunscreens in the EU.

Titanium dioxide is sometimes referred to as a physical UV filter because in a micronised form incorporated into a sunscreen emulsion it works by scattering and reflecting UV so that it does not penetrate the skin. Many products on the market within the EU contain titanium dioxide and elsewhere zinc oxide is also used and iron oxide added to "improve" skin colour by artificially deepening one's tan.

According to Zachariadis, analysis of sunscreen products and cosmetics containing UV filters is needed primarily for the determination of a product's sun protection factor (SPF), which is related to the specific UV filters and their concentrations and for regulatory compliance. Indeed, "there is also necessity to ensure that the concentration levels of metals and oxides are lower than the established limits, since there are several known undesirable dermatological side-effects mainly from organic UV filters," the researchers explain. Despite the existence of regulations controlling such products, there are no official methods to determine the inorganic constituents of sunscreen cosmetics.

The team has now carried out an inductively coupled plasma atomic emission spectrometric (ICP-AES) analysis in a multi-element assessment of several commercially available sunscreen creams and lotions. "The objective was the simultaneous determination of titanium and several minor, trace or toxic elements (aluminium, zinc, magnesium, iron, manganese, copper, chromium, lead, and bismuth) in the final products," the researchers say. They then compared their results with data from atomic absorption spectrometry (AAS) as a reference method.

In the preparation of samples, the researchers tried two alternative pre-treatment procedures. The first was total acid digestion in a closed pressurized vessel prior to introduction of the sample into the plasma. The second involved direct introduction of the sample into the plasma as an emulsified slurry. The second approach was inappropriate for the analysis of several products because of their creaminess, high viscosity, or insolubility. Using the former technique, however, they obtained calculated recoveries of 95.0% for titanium and almost 100% for zinc, while iron recovery was above 100%.

"Most of the commercial preparations that were studied showed generally good agreement to the ingredients listed on the product label," the researchers conclude. However, they also point out that the quantitative composition of the products tested cannot be assessed because the product labels usually do not provide a detailed break down of all ingredients and their concentrations. They also point out that worryingly, their tests consistently revealed the presence of elements not cited in the product formulation, which emphasised the need for a standardised and official testing method for multi-element quality control of these products.