# Key Properties for the Toxicity Classification of Chemicals:
## 1 Introduction  
There is no doubt about the improvements in quality of life that the development of the chemical industry has provided: solvents, paints, pharmaceuticals, cosmetics, additives, and new materials. Chemistry surrounds us and has allowed the development of society as we know it today. However, chemistry has another face closely related to pollution and the health risks associated with the use of chemical substances. The environmental movement has evolved exponentially in recent years; however, it was not until the end of the 1950s that there was a true awareness of the harmful effect that industrialization and the massive and uncontrolled use of chemical substances could cause, not only to the environment but also to humans. The turning point was perhaps in 1962, with the publication of _Silent Spring_ by [PERSON], when the North American population became aware of the serious problem that the lack of control over chemical substances could cause. In her famous book, [PERSON] detailed the effects of certain pesticides on the eggs of various birds, illustrating how dichlorodiphenyltrichloroethane (DDT) and other pesticides could spread through the food chain, causing irreparable damage to the eggs and wildlife. At about the same time, in Europe, panic spread because pregnant women were given a drug called thalidomide, used to reduce the effect of nausea. Thismolecule was administered in its two enantiomeric forms; one of them produced significant defects in newborn babies. In most cases, babies were born without limbs or with significant limb deformities (about 10,000 cases worldwide and 5000 in Germany alone [1]). In both cases, public opinion became aware that the substances in question, designed by the scientists in whom people had placed their trust, were not as safe as they believed, and the unintended and catastrophic consequences began to be realized. For this reason, governments began to regulate the use of chemical substances; legislation was increased to control the manufacture, use, and distribution of chemicals and to establish water, cleaning, and wastewater treatment controls. However, during the 1990s, a new trend arose that considered if it was enough just to regulate and control chemical substances. Was control the only and most effective way to protect humans and the environment from disastrous outcomes? It was concluded that there was another way to pose the problem. Indeed, in 1990, the Pollution Prevention Act was approved. This act regulated the development of the prevention of waste formation at the source. Using various methodologies and techniques, contamination can be prevented, thereby eliminating the need for post-treatment to control chemicals, aligned with Green Chemistry principles [2]. In Europe, a change in the regulatory current also began and was finally established in the form of REACH (Registration, Evaluation, Authorisation, and Restriction of Chemicals). This regulation aims to protect both human health and the environment from chemical substances and mixtures in Europe (European Chemicals Agency, European Commission). REACH is intended not only to mitigate the environmental damage that new compounds can cause but also to protect human health. European companies and industries have been forced to incorporate this type of regulation in their manufacturing and industrialization processes since its implementation (1 June 2007). Nowadays, there are more than 80,000 chemical substances registered for use in Europe [2]. Even if a huge amount of toxicological data has been collected over the years, there are still chemical compounds for which the toxicological potential is unknown. Furthermore, there is generally a high lack of knowledge about the environmental and health implications of many of the compounds already on the market [3]. REACH applies to all substances produced or manufactured in more than one ton per year in the EU. Marketing specifications become higher the higher the production in tons per year is.