Lecture on Toxic Metals: Arsenic, Chromium, Mercury, and Lead

Arsenic

• Properties and Toxicity

  • Arsenic is a toxic metalloid used historically as poison ("poison of kings").
  • Causes acute death and chronic exposure can damage liver, kidneys, and lead to cancer.
  • Multi-organ toxin affecting skin (leading to skin cancer), lungs, kidneys, and nervous system.

• Environmental Presence and Forms

  • Found naturally in rocks, soil, water, air, plants, and animals.
  • Exists in +3 (arsenic trioxide) and +5 (arsenic pentoxide) oxidation states.
  • Inorganic arsenic (bound to non-carbon elements) is more toxic than organic arsenic (bound to carbon).

• Use and Exposure

  • Limited medicinal use; used in pesticides, taxidermy, wood preservation (e.g., Chromated Copper Arsenate - CCA).
  • Main exposure through occupational settings and contaminated water.
  • Chronic water exposure in Bangladesh led to arsenicosis.

Chromium

• Properties and Toxicity

  • Chromium exists as essential trivalent chromium (Cr³⁺) and toxic hexavalent chromium (Cr⁶⁺).
  • Cr⁶⁺ is highly toxic, soluble, and mobile, causing oxidative damage and is a carcinogen.

• Uses and Exposure

  • Used in industrial products: chrome plating, pigments, corrosion protection, and leather tanning.
  • Main exposure occurs via inhalation in occupational settings.

Mercury

• Properties and Environmental Cycle

  • Mercury is unique as a liquid metal at room temperature and exists as elemental mercury (Hg⁰), inorganic mercury (Hg²⁺), and organic mercury (e.g., methylmercury).
  • Environmental mercury comes from natural sources and industrial processes.
  • Inorganic mercury can transform in the environment to more toxic organic forms.

• Exposure and Toxicity

  • Elemental mercury vapor is highly toxic when inhaled, can cross blood-brain barrier.
  • Organic mercury from fish can cross placental barrier, affecting fetus development.

Lead

• Properties and Usage

  • Lead is a malleable metal used for thousands of years, previously in gasoline, paints, and pipes.
  • Found in old paints, toys, candies, and some folk medicines.

• Exposure and Toxicity

  • Absorption of lead is higher in children, leading to developmental issues.
  • Toxic to nervous system, causes reproductive harm, and crosses the placental barrier.
  • Lead poisoning in children can lead to IQ reduction, behavioral issues, and severe developmental impacts.

General Concepts

• Mechanisms of Toxicity

  • Metals often disrupt enzyme activity, mimic essential elements (e.g., arsenic mimics phosphate), and cause oxidative damage.

• Public Health Implications

  • Understanding metal toxicity is crucial for preventing exposure and mitigating health impacts.
  • Emphasis on studying and revisiting key concepts from this lecture for comprehensive understanding.