Iron (ferrous sulphate or iron liquor) is harmful if swallowed.
Large amounts of iron salts can damage wool and other animal fibres, wool fibres weaken and deteriorate over time. Iron is best used with cotton and other plant fibres.
Wash any dyeing equipment and fibre that comes into contact with iron very well to remove any unattached iron particles. Ideally you should keep a dye pot just for iron as even small amounts of iron salts can cause marked colour changes.