Osmium tetroxide is a rare and expensive toxic chemical used as a staining agent for biological samples examined under electron microscopes.
At room temperature, osmium tetroxide sublimates (turns from a solid to a gas without turning to a liquid) and its fumes can be extremely toxic, which can cause irritation to the eyes. This is typically the first symptom of exposure.
As with any toxic chemical substance, direct exposure can cause many adverse health effects. Direct contact on the skin can cause burns, discoloration and blisters. If the eyes are directly exposed, it can damage the cornea and may even turn them black, resulting in blindness.
Inhalation of the osmium tetroxide can cause coughing, shortness of breath, headaches and pulmonary edema, where fluid builds up in the lungs. At high concentrations, this can result in death.
Osmium tetroxide is typically colourless or pale yellow, though it can also be a white crystalline solid that, at room temperature smells like chlorine.
In 2004, a plot to use osmium tetroxide in an improvised explosive device was uncovered in the United Kingdom. Experts said at the time that it wouldn’t have made a good explosive device since once it is heated it changes its chemistry. However, it could have been used as an airborne toxin.