Arsenic
Danger! According to the harmonised classification and labelling (CLP00) approved by the European Union, this substance is toxic if swallowed, is toxic if inhaled, is very toxic to aquatic life and is very toxic to aquatic life with long lasting effects.
Additionally, the classification provided by companies to ECHA in REACH registrations identifies that this substance may cause cancer, may damage fertility or the unborn child, causes damage to organs through prolonged or repeated exposure, is a flammable solid, is self-heating and may catch fire, causes serious eye damage and causes skin irritation.
Source and more information at https://echa.europa.eu/substance-information/-/substanceinfo/100.028.316