Mercury: The invisible toxin

The heavy, silvery-white liquid metal mercury—once valued for its use in medical measuring devices such as thermometers and BP apparatus, in dental fillings, and in various industrial processes—is now recognised as one of the most hazardous substances to human health and the environment.

Almost all of us carry at least trace amounts of methylmercury in our tissues, a reflection of its pervasive presence in our environment. (Elemental mercury is the liquid metal itself, while methylmercury is an organic compound formed when mercury combines with carbon and hydrogen.)

Despite occurring naturally, mercury (chemical symbol Hg, atomic number 80) has become a global threat largely due to unchecked human activity. The World Health Organisation (WHO) lists it among the top ten chemicals of major public health concern.

Mercury, also known as ‘quicksilver’, exists in several forms, but methylmercury—the organic and most toxic form—bioaccumulates in aquatic organisms and enters the food chain, especially through fish and shellfish. Another major source of exposure is inhalation of mercury vapour from broken or leaking medical devices, fluorescent bulbs, or industrial waste. This vapour is rapidly absorbed into the bloodstream and can travel to the brain and other organs. 

Once inside the body, mercury targets multiple systems, with the nervous system especially vulnerable. Exposure can lead to tremors, memory loss, cognitive and motor dysfunction, anxiety, insomnia, and even hallucinations. In pregnant women, mercury crosses the placenta, impairing foetal brain development. Children exposed in utero may suffer developmental delays, reduced IQ, and lifelong learning difficulties.

Mercury’s damage extends far beyond the brain. It can harm the lungs, causing inflammation, bronchitis, and potentially fatal respiratory failure. In the digestive system, it may cause ulcers, bleeding, vomiting, and liver damage. It disrupts cardiovascular health, increasing the risk of arrhythmia, atherosclerosis, and stroke. It affects kidneys, bones, metabolism, and the reproductive system, causing infertility, menstrual irregularities, miscarriages, and low birth weight.

What makes mercury particularly dangerous is its invisibility. It leaves no immediate trace, accumulating silently in the body, making detection difficult and much of the damage irreversible.

Mercury released through coal combustion, mining, industrial waste, and improper disposal of mercury-containing devices (such as thermometers and sphygmomanometers) cycles through air, water, and soil, contaminating ecosystems for decades. In water and soil, it transforms into methylmercury, which is readily absorbed by aquatic life. Through bioaccumulation, smaller organisms store mercury in their body tissues; through biomagnification, it becomes more concentrated at higher levels of the food chain – in large fish, birds, and ultimately humans.

Aquatic ecosystems suffer immensely. Mercury reduces species diversity, disrupts reproduction, delays growth, and causes abnormal behaviour in animals. Top predators, including humans, bear the highest toxic load. Communities dependent on fishing are particularly at risk—both economically and nutritionally.

India’s commitment to the Minamata Convention on Mercury is commendable. Efforts are underway to phase out mercury-based devices, regulate emissions, and promote safer alternatives. However, challenges remain. Mercury thermometers, sphygmomanometers, and amalgam dental fillings are still in use in some places.Awareness is low among healthcare workers, waste handlers, and the general public. Infrastructure for safe disposal is underdeveloped.

Sustainable alternatives are available—digital thermometers, aneroid and digital sphygmomanometers, and mercury-free industrial processes. Bioremediation techniques, which use microorganisms to clean contaminated sites, also show promise.

Addressing mercury pollution requires a multi-sectoral approach. Health professionals must be trained to move away from mercury-containing devices. Industrial standards should be updated to eliminate mercury use. Regulations must be implemented rigorously.

Mercury may be invisible, but its effects are painfully real. As a neuro- and nephro-toxin and persistent pollutant, it is a stark reminder that human and environmental health are deeply intertwined. Eliminating mercury from our lives is not merely a choice—it is essential for a healthier, more sustainable future.

(Naveen is a professor in the Department of Community Health, Division of
Occupational Health at St John’s Medical College, Bengaluru; Akshay is an environmentalist and director of Eco-Watch, Bengaluru)