**1. Introduction**

A cosmetic is any product that is intended to be applied superficially to the human body to keep the treated part in good health. In this process, the cosmetic should not alter the physiological functioning of the body [1]. The use of cosmetics has been practiced since antiquity as apart from cleansing, cosmetics also beautify and alter the appearance hence making the individual more appealing and attractive. There are a plethora of ingredients that were and are used in the formulation of cosmetics. The general intended purposes of cosmetics have not changed throughout centuries and millennia, the formulation of these cosmetics has undergone significant transformations, some of which include the processing and the ingredients used for their formulation. The processing has changed from a domestic/small scale production to cater for a small number of individuals to industrial/large scale production to cater for a wider consumer population with the use of worldwide advertising and social media. On the other hand, the constitution of cosmetics has also changed with time. With industrialization and the use of petrochemically-derived substances, the cosmetic formulation changed from one based on natural products to one which is mainly based on petrochemicals. However, during these past decades, there was a change in the constitution of cosmetics, to include more natural ingredients, due to the great interest and concern by consumers.

As a consequence, the massive production of cosmetic products with a wide range of ingredients, has raised several health and safety concerns. Nowadays, cosmetics are generally regulated [2]. One major concern is that cosmetics overlap in use and functionality with topical medicines. Several regulatory bodies attempted to devise a proper definition for cosmetic products with the intent to segregate cosmetic products from topical medicinal products. Within the European Union (EU), Council Directive 93/35/ EEC [3] amending Council Directive 76/768/EEC, a definition for cosmetic products was laid down in article 1 of the directive. In the first part of the definition, the external body parts which may be treated with cosmetics are mentioned. Other body parts are excluded and this eludes to the understanding that cosmetic products should not be applied to these other body parts. The second part is related to the 'activities' which are allowed for a product to be considered as a cosmetic. These distinguish cosmetics from topical medicinal products which are intended for the control or treatment of conditions or else in making a medical diagnosis [4]. However, whereas topical medicinal products are meticulously scrutinized before their placement on the market, cosmetics do not undergo rigorous testing. Nevertheless, for cosmetic products, the manufacturers, distributors and importers are responsible for the safety of cosmetic products being placed on the market [5]. The latter regulation also states clearly what ingredients are prohibited for their presence in cosmetic products. Amongst the prohibited ingredients several heavy metals are also included. Whereas some metals and their salts are completely prohibited (e.g., tin, arsenic, cadmium, nickel and lead), other metals and their salts are either allowed with a specific limit or else only specific salts for such metals are allowed (e.g., cobalt, chromium, gold, mercury and selenium amongst others). Such additions may not be intentional as the addition of some minerals may originate from a natural source. Heavy metals, such as cadmium (Cd), lead (Pb), nickel (Ni), arsenic (As) and mercury (Hg) were also detected in numerous other raw materials which can be used for the production of cosmetics considered as natural products. These include honey [6], argan oil [7], and olive oil [8] as well as citrus essential oils [9].

Because of this, some authorities also impose limits on the presence of certain metals in cosmetics. For example, The Cosmetic Ingredient Review Expert Panel established by Food and Drug Administration (FDA) in the USA issued limits on As (5 ppm), Pb (5 ppm) and other heavy metals (20 ppm) [10]. The World Health Organisation (WHO) set limits for Pb (10 ppm), Cd (0.3 ppm) and Hg (1 ppm). The EU's limits for Pb, Cd and chromium are 0.5, 0.5 and 1.0 ppm, respectively, while the Canadian authorities set limits for Pb (10 ppm), Cd (3 ppm) and Hg (3 ppm) [11]. However, there tends to be inconsistency in the type of metals and the limits for the metals by different authorities. As a consequence, this lack of harmonization leads to confusion amongst authorities as regulators, several manufacturers as producers and the general population as consumers. Despite all this, several researchers investigated the potential presence of heavy metals in a wide range of products. This review aims at compiling a large number of studies related to the presence of heavy metals in cosmetics and their potential harm in human beings.
