Toxic Metals and Elements
Chapter 6: Toxic Metals and Elements
Learning Objectives: By the end of this chapter, the reader will be able to:
Name five heavy metals that pose significant health hazards to humans.
State mechanisms for human exposure to toxic metals, differentiating between acute and chronic pathways.
Distinguish between essential trace metal levels, which are beneficial, and toxic levels, which cause adverse health effects.
Describe common occupational settings in which workers are exposed to elevated levels of toxic metals.
Discuss effective methods for the prevention and mitigation of exposure to toxic metals in various environments.
Introduction:
Human exposure to toxic metals is a significant public health concern, stemming from natural processes and anthropogenic activities like industrialization.
This chapter focuses on the diverse health effects of toxic metals, particularly on vulnerable populations such as women and children due to their heightened susceptibility.
It will cover metallic compounds, environmental sources, toxicity mechanisms, and health effects.
Future discussions will include other environmental hazards like pesticides and organic chemicals.
Significance of Heavy Metals:
Heavy metals (e.g., lead, mercury, nickel) and other metallic compounds (e.g., aluminum, iron, tin) are ubiquitous, persistent environmental pollutants with long half-lives.
Sources of human exposure include:
Natural deposits: Geogenic sources leaching into groundwater (e.g., arsenic).
Smelter emissions: Industrial refining releases particulate matter into air, soil, and water.
Contaminated food/water: Agricultural practices, industrial discharge, and pollution affect crops, livestock, fish, and potable water.
Occupational exposure: High risks for workers in mining, manufacturing, construction, and electronics.
Lead:
Harms child neurodevelopment, causing irreversible cognitive impairment, behavioral issues (ADHD), reduced IQ, and learning disabilities. Also affects renal, cardiovascular, and hematopoietic systems.
Persists in older homes from lead-based paint dust and lead pipes/solder in plumbing, contaminating drinking water.
Flint, Michigan water crisis: Lead leached from aging pipes due to inadequate corrosion control, exposing many, especially children.
Mercury:
Widely contaminates fish as toxic organic methylmercury, which bioaccumulates and biomagnifies up the food chain (e.g., tuna, swordfish).
Pregnant women are advised to avoid high-mercury fish due to severe damage to fetal brain development and the nervous system.
Priority List of Hazardous Substances:
The ATSDR, under CERCLA (Superfund), publishes a list of hazardous substances posing significant health threats.
Top substances consistently include arsenic, lead, mercury, and cadmium, reflecting their widespread occurrence, high toxicity, and exposure potential.
Prioritization considers occurrence frequency at contaminated sites, known/suspected toxicity, and potential for human exposure.
Overview of Sources and Effects of Exposure:
Metals are classified into three toxicological categories:
Major Toxic Metals: High atomic weight, no physiological function, generally toxic at low levels, bioaccumulative (e.g., arsenic, lead, mercury, cadmium).
Essential Metals (potential toxicity): Vital in trace amounts but toxic at higher concentrations (e.g., copper, zinc, iron).
Metals Used in Medical Therapy: Utilized therapeutically with inherent risks (e.g., aluminum, lithium, platinum).
Major Toxic Metals: Persist in the environment and biological systems. Arsenic is a potent carcinogen (skin, lung, bladder, kidney cancer), found in groundwater and industrial byproducts. Bioaccumulation occurs in fish and shellfish, transferring to humans.
Sources of Exposure:
Inhalation: Breathing dust/fumes in occupational settings (mining, welding) or from environmental sources (vehicle exhaust, industrial emissions).
Dermal Contact: Skin absorption from contaminated soil, cosmetics, or water.
Ingestion: Primary route via contaminated food (vegetables, fish), drinking water, or incidental soil/dust intake.
Environmental Media: Air, soil/dust, water, and food biota are main routes.
Children are highly susceptible via lead paint and contaminated food/water due to hand-to-mouth activity and higher absorption rates.
Effects of Exposure:
Acute exposure: Immediate, severe symptoms affecting gastrointestinal (nausea, vomiting) and neurological systems (tremors, seizures).
Chronic low-level exposure: Subtle, insidious symptoms over time, including cognitive impairment, developmental delays, kidney damage, cardiovascular issues, and cancers.
Vulnerable Groups: Women and children show different susceptibilities. Mercury crosses the placental barrier, damaging fetal brain (Minamata disease). Lead affects women's bone density and can transfer to the fetus. Chronic childhood exposure is linked to developmental disabilities, learning issues, and reduced cognitive function.
Specific Toxic Metals:
Arsenic: In water/soil (Bangladesh, India, U.S.), industrial pollution, pesticides, some foods (rice, seafood). Strict drinking water limit 10 ext{ μg/L}, known carcinogen, causes skin lesions.
Beryllium: Primarily occupational hazard (aerospace, electronics), inhalation causes chronic beryllium disease (CBD), a debilitating lung condition. Known human carcinogen (lung cancer).
Cadmium: Byproduct of mining (zinc, lead, copper), in batteries, pigments, plastics, tobacco smoke. Causes severe kidney damage, bone demineralization, linked to lung, prostate, kidney cancers.
Chromium: Cr(III) is essential, but hexavalent chromium (Cr(VI)) is highly toxic, corrosive, and a potent carcinogen via inhalation (lung cancer). Causes skin ulcers, kidney damage (e.g., Hinkley, CA contamination).
Mercury: Causes extensive neurological damage. Forms: elemental vapor (brain, kidneys), inorganic (kidneys), organic methylmercury (central nervous system, fetal brain). Minamata disease (Japan, 1950s) was from methylmercury poisoning.
Lead: Severe effects on child health (central nervous system, kidneys, blood-forming system). Historical use in gasoline, paint, glazes, plumbing necessitated significant policy changes (phasing out leaded gasoline/paint).
Essential but Potentially Toxic Metals:
Copper: Essential for enzymatic functions, iron metabolism. Excess causes GI distress, liver damage, neurological issues (Wilson's disease), often from contaminated water or supplements.
Zinc: Vital for immune function, wound healing. High exposure causes GI distress, respiratory irritation (metal fume fever), copper deficiency, from industrial fumes or excessive supplementation.
Iron: Essential for oxygen transport. Deficiency causes anemia. Excess accumulates in organs (liver, heart, pancreas), causing multi-organ damage (hemochromatosis) and is worsened by over-supplementation.
Metals for Medical Use:
Aluminum: Found in medications (antacids, antiperspirants, vaccines). Links to Alzheimer's disease are debated and under research.
Lithium & Platinum: Lithium compounds treat bipolar disorder. Platinum compounds (cisplatin, carboplatin) are powerful chemotherapy agents, inhibiting cancer cell proliferation.
Conclusion:
Toxic metals are pervasive environmental contaminants, posing complex public health challenges due to their persistence, bioaccumulative potential, and diverse toxic mechanisms.
Public health efforts must prioritize strategies to reduce human exposure (regulation, remediation, education) and prevent environmental contamination.