Toxic Metals Notes

Learning Objectives

• Identify five heavy metals that pose health hazards to humans.

• Identify mechanisms for exposure of humans to toxic metals.

• Distinguish between essential and toxic levels of trace metals.

• Identify occupational settings in which workers are exposed to toxic metals.

• Discuss methods for prevention of exposure to toxic metals.

Introduction to Toxic Metals

Characteristics of Metals

• Form cations (positively charged ions) when dissolved in solution.

• Form bases in solution.

• Good conductors of heat and electricity.

• Most are crystalline solids at room temperature with a shiny, metallic luster.

• Most are malleable and ductile.

• Most are hard and have high physical strength.

• Most are denser than water.

CERCLA Priority List of Hazardous Substances

National Priority List (NPL)

• Definition:

  • "The National Priorities List (NPL) is the list of sites of national priority among the known releases or threatened releases of hazardous substances, pollutants, or contaminants throughout the United States and its territories." – EPA.gov

• Jointly managed by the Agency for Toxic Substances and Disease Registry (ATSDR) and EPA.

• Outlined by CERCLA Act.

• Superfund Program provides funding for cleanup of NPL sites.

ATSDR Substance Priority List

• Also known as CERCLA Priority List.

• NOT a rank ordering of ALL toxic substances.

• IS a ranked order of hazardous substances that are most commonly found on NPL sites.

• One of the best resources for understanding environmental contamination, and for data gathering.

• Includes substances with a known or suspected toxicity AND that have a potential to cause an exposure to humans on NPL sites.

• Substances may have lower toxicity but found at higher frequencies at NPL sites.

• Generally updated every 1-2 years.

Substance Priority List (2022 vs 2007)

Sources of Exposure to Metals

Types of Metals

Major Toxic Metals

• Typically associated with multiple effects: arsenic, beryllium, cadmium, chromium, lead, mercury, nickel.

Therapeutic Metals

• Elements used in medical treatments to treat or cure illnesses or injuries. Examples include aluminum, lithium, platinum.

Essential Metals

• Elements (think "vitamins and minerals") necessary to sustain life but with potential for toxicity, e.g., copper, iron, zinc.

Exposure to Toxic Metals

Where It Can Come From

• Air

• Soils/dust

• Water: drinking, rain, ground

• Food

• Flora/Fauna

Modes of Exposure

• Inhalation: dusts, metal fumes, vapors.

• Absorption: dusts.

• Ingestion: dusts, fumes, food, water.

• Injection: metal shards.

Bioaccumulation vs Biomagnification

• Bioaccumulation: Accumulation of substances within an organism.

• Biomagnification: Increasing concentration of substances up the food chain, increasing risk of metal toxicity in predators.

• Many heavy metals, e.g., mercury, demonstrate both processes.

Exposure Risk

• The most common and highest excessive exposures will come through occupational settings.

• Electronic waste in less developed countries increases occupational hazards.

• Lower-level exposures typically are seen in general ambient circumstances, but non-occupational exposures can sometimes be similarly large.

Effects of Toxic Metals

Acute Toxicity

• Acute poisoning effects generally show up rapidly – within minutes to one hour.

• Route of exposure is usually inhalation or ingestion.

• Symptoms depend on the route of exposure but most commonly include:

  • Gastrointestinal effects: vomiting, diarrhea, stomach pain.

  • Neurological effects: headaches, convulsions, reduced motor control; can even affect the ability to breathe.

Effects of Chronic Exposure

• Chronic exposures, especially at low levels, can be hard to differentiate from other chronic medical conditions.

• Chronic metal toxicity has been associated with symptoms such as reduced cognition, learning impairments, and lethargy.

• An increased risk of developing various types of cancer.

Carcinogenic Potential

• IARC Classification System:

  • Group 1: The agent is carcinogenic to humans.

  • Group 2A: The agent is probably carcinogenic to humans.

  • Group 2B: The agent is possibly carcinogenic to humans.

  • Group 3: The agent is not classifiable as to its carcinogenicity to humans.

  • Group 4: The agent is probably not carcinogenic to humans.

• Arsenic, beryllium, cadmium, chromium, and nickel are classified as Group 1 carcinogens.

Heavy Metals

Definition

• High atomic weight.

• Specific gravity is 5+ times that of water.

  • Examples: nickel, cadmium, lead, mercury.

• Gender differences in hormones and metabolic processes may associate with differing effects from heavy metal exposures between men and women.

• Children may demonstrate more significant responses to metal toxicity due to their smaller body weight and proportionally larger consumption of food as an exposure source.

• Rapid growth in children can amplify risks and contribute to neurological deficits, learning disorders, memory impairment, and behavioral issues.

A Case Study: Methylmercury Toxicity

• Samples of brain tissue from Minamata disease patients illustrate the devastating effect of high methylmercury exposure.

  • 7-year-old child: Sample shows extensive damage after 4 years of exposure.

  • 8-year-old child: Sample shows damage after 2.75 years of exposure.

  • 30-year-old man: Sample shows lesser damage compared to younger subjects.

Risk to Fetuses and Infants

• Fetuses and infants are at the greatest risk from heavy metal exposures due to impaired system development.

• Increased risks of certain cancers may occur, with potential fatal outcomes at lower proportional doses.

  • Two heavy metals, lead (Pb) and mercury (Hg), can cross the placental barrier and are associated with fetal neurological damage.

Dose-Response Curve

• Illustrates the relationship between the dose of a chemical and a toxic response.

• No observable adverse effect level (NOAEL) and lowest dose at which an adverse effect is observed (LOAEL) are indicated.

Major Toxic Metals

Seven Major Toxic Metals

• Naturally occurring with exposures in both residential and occupational settings.

• Six are considered “heavy metals” by textbook definitions; beryllium is not, yet it is still studied for its toxicity.

• Major Heavy Metals:

  1. Arsenic

  2. Cadmium

  3. Chromium

  4. Lead

  5. Mercury

  6. Nickel

Specific Gravities of Selected Metals

Health Effects of Arsenic

• Usage:

  • Used in pesticides, wood preservatives, other manufacturing processes; can be a by-product of metal refining.

  • Historically used medicinally and for cosmetic purposes.

• Exposure Routes:

  • Most exposures are through ingestion and inhalation.

• Toxicity:

  • Varies according to chemical form (inorganic vs organic); inorganic compounds are more toxic.

  • Drinking Water Standard (EPA, 2001): 10 µg/L (previously 50 µg/L).

• Potential Health Effects:

  • Cancer:

  • If ingested: skin, bladder, kidney, liver cancers.

  • If inhaled: lung cancer.

  • Melanosis: skin darkening.

  • Cardiovascular diseases, cerebrovascular diseases, diabetes.

  • Reproductive disorders, including spontaneous abortions, stillbirths, and preterm births.

Beryllium (Be)

• Not a heavy metal by most definitions but still considered hazardous in industrial hygiene.

• Common exposures primarily through inhalation.

• Associated with "Chronic Beryllium Disease," cardiovascular deficits, and cardiomyopathy.

• Released into the environment through burning coal affecting agricultural products.

• Classified as a “Probable Carcinogen” by the EPA and known by other agencies.

• OSHA reduced occupational exposure from 2 µ/m³ to 0.2 µ/m³ for an 8-hour average.

Cadmium (Cd)

• Common Non-Occupational Exposures:

  • Inhalation through tobacco/cigarette smoke.

  • Ingestion from shellfish and mushrooms (notably linked to bioaccumulation).

• Common Occupational Exposures:

  • Manufacturing: Ni-Cd batteries and paints.

  • Zinc smelting and metalworking.

• Symptoms and Toxicity:

  • Osteoporosis, kidney damage, hypertension, cardiovascular diseases.

  • “Itai-Itai” disease: caused by industrial dumping into local rivers, resulting in severe bone fractures and cardiovascular damage.

Chromium (Cr)

• Common Forms:

  • Chromium (III): essential nutrient.

  • Chromium (0): elemental form.

  • Chromium (VI): used in manufacturing and is a carcinogen.

• Toxic Effects:

  • Ingestions can lead to indigestion and organ damage (liver, kidney).

  • Absorption can cause skin ulceration, while inhalation leads to respiratory injuries.

• Erin Brockovich & Hinkley Case:

  • Landmark legal case over Chromium VI contamination by PG&E in California; resulted in a $333 million settlement, highlighting challenges in environmental remediation.

Mercury (Hg)

• Historically used as a purgative and treatment for syphilis.

• Symptoms of Mercury Poisoning:

  • Neurological deficits: tremors, ataxia, loss of concentration, mood swings, memory loss.

  • Dermatitis and vision deficits.

• Minamata Disease:

  • Caused by industrial discharge of mercury into water, leading to severe neurological damage through bioaccumulation in the food chain.

Lead (Pb)

• Exposure Sources:

  • Leaded gasoline, water from lead-soldered pipes, lead-based paints, ceramics with lead materials, metalworking, firearm ranges.

• Toxic Responses:

  • Neurological deficits, anemia, and blood disorders; among the most common public health problems in pediatrics in the US.

Nickel (Ni)

• Ubiquitous in nature and commonly associated with metal items in appliances, batteries, and jewelry.

• Chronic exposure leads to contact dermatitis, cardiovascular and renal diseases, fibrosis of the lungs, and associated cancer risks.

Essential Metals

Definition

• Essential metals are necessary to sustain life; however, in excess amounts, they can cause toxic responses.

• “Dose Makes the Poison!”

Key Essential Metals

Copper (Cu)

• Supportive role in enzymatic reactions, collagen and elastic formation, iron metabolism, immune function, and neurological health.

• Found in electrical wires, pipes, and as a preservative.

• Exposure Routes: Inhalation or ingestion from water pipes or industrial releases.

• Toxicity: Excessive amounts can lead to gastrointestinal disturbances, respiratory issues, and at extreme levels, liver/kidney damage or death.

Zinc (Zn)

• Found in all earth media, thought to aid in reducing infections, childhood growth, and healthy pregnancy.

• Occupational uses include anti-rust coatings and battery components.

• Toxicity: Causes gastrointestinal issues, anemia, and pancreatic damage, as well as “Metal Fume Fever” from welding.

Iron (Fe)

• Vital for cells and oxygen transport, with accidental ingestion of supplements being a common poisoning source.

• Toxicity: Can lead to gastrointestinal disturbances, metabolic acidosis, liver/kidney damage, and endocrine dysfunction.

Metals Used in Medical Therapies

Therapeutic Medicines

• Aluminum (Al): Used in medications for heartburn and kidney issues, with concerns regarding cognitive disorders like Alzheimer’s.

• Lithium (Li): Treatment for depression and bipolar disorder.

• Platinum (Pt): Used in medical equipment and chemotherapy drugs (e.g., cisplatin).

Preventing Toxic Metal Exposure

Prevention Strategies

• Source Reduction: Limit releases into the environment (e.g., banning leaded gasoline, proper disposal of electronic waste).

• Occupational Safety: Implementation of personal protective equipment (PPE), improved ventilation, and hygiene practices.

• Food Safety: Guidance on fish consumption and food washing practices.

• Water Safety: Lead-free plumbing, water filtration, and regular flushing of taps.

• Contaminated Sites: Cleanup efforts, such as those by Exide Technologies.

• Public Education: Raising community awareness for safer practices.