In its liquid form or as an alloy, mercury has little or no deleterious effect on human health. However, mercury vapours and most mercury compounds are very harmful to human health. The initial effects of mercury poisoning were seen in the 19th century in felt hat makers who almost always showed symptoms mimicking mental illness after a long period at work in their profession. The cause was soon traced to the mercury used in treating the felt.
Today, mercury exposure is often from the workplace as it is still released in certain industrial processes, such as smelting. Mercury also accumulates in the environment and large fish (particularly carnivorous fish like swordfish) should be monitored for mercury levels. Accidental exposure can result from products containing mercury, such as old thermometers, old barometers, and fluorescent light bulbs.