# Key Properties for the Toxicity Classification of Chemicals:
## 4 Chemical's Toxicity Assessment Studies Aligned with Regulatory Requirements
### Physicochemical Properties  
Most of the found manuscripts are classified in environmental sciences, ecology, chemistry, toxicology, zoology, biochemistry and biology, pharmacology, phylacyl, public environmental occupational health, marine freshwater biology, agriculture, or water resources, among others, are the most typical research areas. Of course, there are numerous scientific studies whose objectives are related to obtaining safer substances and processes or the rational design of chemicals, with a green, sustainable, and health promotion approach. However, these represent a small percentage (approximately 25%) of the totality of the studies about physicochemical properties from 2007 to date. On the other hand, it should be noted that there is a large group of physicochemical properties available that are provided by the scientific community, with the aim of being used to characterize the risk associated with the use of these substances or their sustainability and health nature, which are not required for the REACH regulation. For example, the dissociation constant (pKa), the overall persistence or half-life, is a measurement related to degradation and formation of byproducts [38; 39]. Properties such as organic carbon sorption coefficient (log Koc) or distribution coefficient (log Dow) are measured for their connection with the capacity to remove contaminants in natural matrices [38]. Typical thermophysical properties are also used: refractive index, density, specific heat capacity, and viscosity [40] as well as properties not so common such as contact angle [41], total soluble solids, titrable acidity and firmness [42], or long-range transport potential [39]. All properties aimed at characterizing chemical substances from the point of view of chemical and molecular analysis are also important for environmental and human health risk minimization. Thus, we find numerous studies that carry out spectroscopic measurements [43; 44], microscopy [45], X-ray diffractometry [46], or elemental chemical analysis [45] in order to analyze the environmental impact of certain substances or justify their safe use. It is worth mentioning that, for the environmental risk evaluation of nanomaterials, other types of properties are measured and analyzed. For instance, properties such as particle diameter, specific surface area, crystallinity, weight and weight loss, transformation rate, zeta potential, or Hamaker constant are normally measured and/or predicted [47; 48; 49; 50]. For this type of material, surface chemistry, morphology, and sedimentation behavior are factors of importance and provide important information when analyzing the environmental impact of nanomaterials. Other properties that are also usually measured when evaluatingthe green character of nanomaterials are those related to the ability of the particles to form aggregates (coating processes or state of aggregation) [48]. In fact, critical aggregation concentration is another property that is used for evaluating the environmental impact of chemicals in general [51; 52]. This property has been related to toxicity, bioavailability, and the ability of molecules to cross biological membranes [52; 53]. It is also remarkable that, to evaluate the potential risk of chemical substances in soil, very different properties are used, which are not required by REACH. For example, sorption degradation and mobility potential [54], rates of degradation [55], soil sorption coefficient [56], pH [55; 57; 58], electrical conductivity [57], soil aggregate stability, and soil organic matter [58] are very commonly used. Finally, there is another important group of physicochemical properties that, although they are not used directly for the environmental or risk assessment of chemical substances, are measured indirectly since they are key properties for the development and application of predictive methods of impacts on the environment or toxicity, as well as Quantitative Structure Activity Relationship (QSAR) or Quantitative Structure Property Relationship (QSPR) methods.