Cobalt(II) chloride (CoCl2) was very often used as a color indicator in the past. When the silica gel has bound water, this indicator changes color from blue to pale pink to whitish; the hexaaqua complex ([Co(H2O)6]Cl2) is formed. On drying, this process is reversible.
To this day, aeration dryers with “blue gel” are used. And this despite the fact that cobalt(II) chloride is considered to be carcinogenic as well as teratogenic and possibly mutagenic. The substance was again included in the candidate list of substances of very high concern (SVHC) by the ECHA (European Chemicals Agency) on October 28, 2008. The entry was amended on June 20, 2011 to include the reproductive toxic properties.
Taking this into account, the EEA (Electricity Engineers’ Association of New Zealand), for example, in a handling recommendation issued to its members in April 2013, recommends the use of silica gel orange to minimize risks.