Why are they hazardous?

Mercury is toxic by inhalation and ingestion. Repeated exposure may cause cumulative poisoning affecting the nervous system with symptoms such as tremors, irritability, nervousness and memory loss. Soluble inorganic compounds of mercury have a corrosive effect on skin.

The vapour from elemental (liquid) mercury is invisible, odourless and without detectable taste. Any source of mercury that is open to air will slowly evaporate and, if allowed to accumulate to harmful levels, will pose a risk of exposure if inhaled. Historically, hat makers who used mercuric nitrate in fur felting showed symptoms of mercury exposure, hence the expression ‘as mad as a hatter’.