Antimony
Danger! According to the classification provided by companies to ECHA in REACH registrations this substance may damage fertility or the unborn child, is suspected of causing cancer, may cause harm to breast-fed children, may cause damage to organs through prolonged or repeated exposure and is harmful to aquatic life with long lasting effects.
At least one company has indicated that the substance classification is affected by impurities or additives.
Source and more information on the European Chemical Agency's website