Other similarly formed databases such as PubChem, ChemSpider, NOR-MAN SusDat, and US-EPA CompTox act as hubs where the data from different chemical registries is gathered, curated and made available for public use. While these larger databases are more generic and tend to have different chemical families, there are also more specialized chemical databases such as drug bank, 62 human metabolome database, FORIDENT. In addition to the literature based chemical databases, there are also national and international chemical registries with mainly regulatory focus. For example the European Chemical Agency (ECHA) formed in 2007 is the registry of chemicals used, imported, and/or exported into the European Union. As these chemical databases are meant for regulatory purposes, they also include information regarding the volume of production/use as well as biological activity (e.g. toxicity) and physiochemical properties. However, these databases may may have different volume of production/use registration thresholds. Furthermore, these databases rarely include the transformation products of human-made chemicals, unless they are being actively produced and used for other purposes. However, these databases are limited to human-made chemicals, their size is increasing by around 1500 new structures a year. Not all human-made chemicals, registered or not, are part of the exposome chemical space, due to their total volume of production, physiochemical properties, and use type. For example, the potential of exposure to a chemical with a very small volume of production may be very low as this chemical once released into the environment is infinitely diluted. Between 2010 and 2012, Howard and Muir published three very influential manuscripts in which they reported lists of high priority chemicals to be further studied. In those studies the authors investigated all the existing North American chemical databases and selected the chemicals with a volume of production larger than 1 ton a year. This threshold was set to assure the environmental detection of these chemicals. Additional filtering (i.e. chemical prioritization) based on physiochemical properties and expert knowledge were employed to narrow down these chemicals to those pertinent to the environmental and human exposome.