Toxicity refers to the potential of a substance to cause harm when it interacts with the body. In the context of cosmetics, toxicity is evaluated by determining the concentration at which an ingredient becomes harmful to human health. Regulatory authorities like the European Chemicals Agency (ECHA) and the U.S. Food and Drug Administration (FDA) set stringent guidelines (The EU Cosmetic Regulation (1223/2009) to ensure that ingredients in cosmetics are used at safe levels [1]. These agencies evaluate the toxicological profiles of substances through extensive studies on both local effects—such as skin sensitization and irritation—and systemic risks, including mutagenicity, carcinogenicity, and reproductive toxicity [2].

Certain substances, however, are entirely prohibited or restricted in cosmetic formulations due to their inherent toxicity, even at low concentrations. For example, lead, mercury, and formaldehyde are either banned or tightly regulated because of their potential to cause serious health issues such as neurotoxicity, organ damage, and carcinogenicity. Heavy metals are absorbed into the bloodstream and accumulate in organs such as the kidneys, resulting in organ toxicity [3]

Allergenicity is the potential of an ingredient to provoke an allergic reaction in some individuals. Unlike toxicity, which is dose-dependent, allergic reactions are typically idiosyncratic and do not always correlate with the amount of exposure. This means that even a tiny amount of a particular ingredient can trigger an immune response in a sensitized individual, while others may tolerate the same substance without any issues.

Currently, there are three hypotheses in the literature that try to explain its pathophysiology, namely the disruption of the epidermal-barrier function, enhanced immune responsiveness, and neurosensory dysfunction [4]. Common allergens in cosmetics include fragrances, conditioning agent, surfactant, preservatives, botanical derivative, UV filter, antioxidants and ph adjusters. Fragrance mixes are the most frequent culprit, with studies indicating that approximately 1.9% of the population may be allergic to their components [5].

It’s important to note that no ingredient is universally allergenic. An individual's genetic makeup, immune system, and previous exposure to certain substances all play a role in whether or not they develop an allergy.

As Paracelsus eloquently put it, "All things are poison, and nothing is without poison; the dose alone makes a thing not a poison." This dose-response relationship is crucial when discussing the safety of cosmetic ingredients. For example, vitamin C (ascorbic acid) is a highly valued ingredient in skincare for its antioxidant and brightening properties. At low to moderate concentrations (5-17%), it is generally safe and effective for most skin types. However, when used at excessively high concentrations, vitamin C can cause skin irritation, redness, and even chemical burns [6,7].

Similarly, botanical extracts and oils, while often considered gentler alternatives to synthetic ingredients, can also exhibit dermatologial adverse when used in high doses. Tea tree oil, celebrated for its antimicrobial properties, can cause severe skin reactions, including blisters and burns, if used undiluted. Another example is salicylic acid, a popular beta-hydroxy acid used to treat acne. At low concentrations (0.5-2%), it is safe and beneficial for most users, but higher doses can lead to skin peeling, dryness, and increased sensitivity to sunlight.

To ensure the safety of cosmetic products, regulatory bodies set strict rules on ingredient concentrations and testing. In the European Union, the Cosmetic Products Regulation (EC) No 1223/2009 lists banned substances and regulates safe concentrations of preservatives, colourants, and UV filters. The U.S. FDA takes a similar approach, overseeing safe use of ingredients like sunscreens, retinoids, and alpha-hydroxy acids.

Cosmetic products undergo rigorous in vitro and in vivo testing to assess toxicity, irritation, and allergenicity before reaching the market. Additionally, a list of 26 allergens (commonly found in fragrances, such as limonene, linalool, and geraniol) must be clearly listed on product labels if their concentration exceeds 0.001% in leave-on products (e.g., creams) or 0.01% in rinse-off products (e.g., shampoos). Despite these safeguards, it's important for consumers to perform patch tests when trying new products to reduce the risk of allergic reactions.

Concerns about toxicity and allergenicity in cosmetics are valid, but these risks depend on dosage, concentration, and individual sensitivity. Regulatory bodies ensure safe use within limits, but personal responsibility is key. By staying informed, testing new products cautiously, and understanding dosage, consumers can enjoy cosmetics while minimising risks.

1. Manful, M.E.; Ahmed, L.; Barry-Ryan, C. Cosmetic Formulations from Natural Sources: Safety Considerations and Legislative Frameworks in the European Union. Cosmetics 2024, 11, 72. https://doi.org/10.3390/cosmetics11030072 

2. EC 1223/2009; The European Parliament and the Council of the European Union Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on Cosmetic Products. European Union: Brussels, Belgium, 2009; Volume 10, pp. 1–376.

3. Arshad H. et al. Evaluation of heavy metals in cosmetic products and their health risk assessment //Saudi Pharmaceutical Journal. – 2020. – Т. 28. – №. 7. – С. 779-790.

4. Misery L., Loser K., Ständer S. Sensitive skin //Journal of the European Academy of Dermatology and Venereology. – 2016. – Т. 30. – С. 2-8.

5. Diepgen TL, Ofenloch R, Bruze M, Cazzaniga BS, Coenraada P-J, Goncalo EM, et al. Prevalence of fragrance contact allergy in the general population of five European countries: A cross-sectional study. Br J Dermatol. 2015;173:1411-19.

6. Caritá A. C. et al. Vitamin C: One compound, several uses. Advances for delivery, efficiency and stability //Nanomedicine: Nanotechnology, Biology and Medicine. – 2020. – Т. 24. – С. 102117.

7. Writer C. I. R. Safety Assessment of Ascorbic Acid and Ascorbates as Used in Cosmetics. – 2024.