Toxicity of Chemical and Physical Agents

Slide 1 · Toxicity of Chemical

and Physical Agents

Exposure • metabolism • molecular injury • susceptibility

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Explain why toxicity depends on dose, route, duration, and host susceptibility.

Trace xenobiotics from environmental compartments to absorption, tissue distribution, metabolism, storage, and elimination.

Distinguish detoxification from bioactivation and explain phase I, phase II, and CYP450 roles.

Apply the framework to source-described toxic exposures and recognize the limits of population exposure thresholds.

Slide 3 · Toxicology: What does the discipline study?

Agents and exposure

Distribution of poisons within a living organism

Chemical agents including drugs and pollutants

Physical hazards such as radiation and heat

Biologic consequences

Molecular and cellular mechanisms of injury

Organ-specific functional and structural effects

Variation among exposed persons

Toxicology connects exposure, fate, mechanism and organism-level response.

Slide 4 · Chemical and physical hazards are related, not identical

Chemical agent

Enters by skin, lung or gastrointestinal tract

May be redistributed, biotransformed or stored

A metabolite may be more toxic than the original compound

Physical agent

Transfers energy or disturbs tissue homeostasis

Includes heat and ionizing radiation

Magnitude, duration and vulnerable tissue affect injury

The shared theme is exposure → biologic response; Metabolic activation is mainly a chemical-agent concept.

Slide 5 · A poison is defined by exposure, not by name alone

Agent

Amount reaching tissue

Biologic effect

The concept of poison is fundamentally quantitative: A substance may be useful at one exposure and harmful at another.

Dose cannot be interpreted without exposure duration, route, target tissue, and susceptibility.

Therapeutic chemicals illustrate the narrow or broad separation between benefit and harm.

Slide 6 · Dose does not equal delivered tissue dose

Administered or encountered

Amount in air, water, food or on skin

Initial route of contact

Duration and pattern of exposure

Biologically available

Fraction absorbed and distributed

Extent of metabolic conversion

Concentration and persistence at sensitive sites

The tissue dose is influenced by toxicokinetics, not just the external concentration.

Slide 7 · Why population “safe levels” have limitations

Permissible exposures support comparison of populations and risk estimation.

Age, genetic predisposition and organ sensitivities modify the effect of the same exposure.

Mixed pollutants may interact, so a single-agent threshold cannot describe every combination.

A regulatory cutoff does not prove that every individual below it is unharmed.

Slide 8 · Four sources of interindividual variation

Age

Developmental and age-associated physiology can change susceptibility.

Genetics

Enzyme polymorphisms alter toxicant metabolism.

Tissue state

Different organs and individuals have different vulnerabilities.

Co-exposure

Drugs, environmental chemicals and mixed pollutants can modify effects.

The same nominal environmental exposure does not imply the same biologic outcome.

Slide 9 · From pollutant exposure to disease risk

Environmental concentration

Individual internal dose

Target-organ response

Observed disease

External measurements are surrogates for biologically effective exposure.

Modifiers can act at uptake, metabolism, distribution or cellular repair.

Risk estimates describe populations; Diagnosis requires patient-specific evidence.

Slide 10 · Xenobiotics are exogenous chemicals

Xenobiotics enter the body from the air, water, food and soil.

The term includes pollutants, solvents, drugs and other externally derived chemicals.

A xenobiotic is not automatically toxic: Dose, chemical identity and biologic handling determine harm.

The Robbins exposure schema follows environmental source → uptake → tissue fate.

Slide 11 · Three portals of xenobiotic entry

Environmental agents

Lung / GI tract / skin

Absorption or local action

Airborne agents reach the body through respiration.

Chemicals in food or water may be absorbed through the gastrointestinal tract.

Skin contact provides a separate absorption pathway; Portal and physicochemical properties both matter.

Slide 12 · Route of entry can predict initial tissue exposure

At the portal

Local airway epithelial injury after inhalation

GI mucosal contact after ingestion

Cutaneous injury following skin contact

After absorption

Blood carries agent to distant tissues

Systemic toxicity may follow any portal

Metabolism can alter the active species

Portal toxicity and systemic toxicity are not mutually exclusive.

Slide 13 · Inhalation: Direct contact and systemic uptake

Pollutants in inspired air contact conducting airways and potentially alveoli.

Local toxicity depends on the identity and properties of the inhaled substance.

Absorbed agents may travel through the bloodstream to organs beyond the lung.

The outdoor-air examples later in this chapter illustrate local oxidant injury and systemic asphyxiation.

Slide 14 · Ingestion: The gastrointestinal entry route

Contaminated food and water can deliver xenobiotics to the GI tract.

Some agents act at the contact site; Others enter the blood and act remotely.

The fate of absorbed material depends on distribution, metabolic conversion and excretion.

Chemical identity matters more than the route alone when anticipating target-organ injury.

Slide 15 · Skin contact: A distinct pathway of absorption

Direct cutaneous exposure can cause injury at the site of contact.

Some chemicals cross the skin barrier and enter systemic circulation.

Lipid-soluble chemicals may traverse cellular membranes readily.

When evaluating exposure, document which surface contacted the agent, not merely the agent name.

Slide 16 · Comparison of environmental entry routes

Entry route

Typical environmental source

First contacted tissue

Inhalation

Airborne gases or particles

Respiratory tract

Ingestion

Contaminated water or food

Gastrointestinal mucosa

Skin contact

Chemicals on exposed surfaces

Skin / cutaneous barrier

Regardless of route, absorbed xenobiotics may produce remote systemic effects.

Slide 17 · Systemic distribution begins after absorption

Entry barrier

Bloodstream

Remote tissue

Response

A chemical may act locally before—or without—appreciable systemic distribution.

Bloodborne agents can reach multiple organs with different sensitivities.

Subsequent storage, conversion and elimination alter the intensity and duration of exposure.

Slide 18 · Lipid solubility changes xenobiotic behavior

Many solvents and drugs are lipophilic.

Lipophilicity facilitates transport in blood associated with lipoproteins.

It also favors movement across cell membranes into intracellular compartments.

Thus distribution is a chemical-property-dependent process rather than simple uniform mixing.

Slide 19 · Why lipophilic substances readily reach cells

Lipophilic xenobiotic

Blood / lipoprotein transport

Membrane penetration

Intracellular access

The lipid-rich plasma membrane permits entry of many lipophilic compounds.

Transport into a cell does not itself establish that the compound is toxic.

Biotransformation after entry may detoxify the compound or form damaging metabolites.

Slide 20 · Distribution is not the same as accumulation

The bloodstream distributes a xenobiotic among tissues.

Some agents persist in reservoirs such as bone, adipose tissue and brain.

Others are excreted or converted into metabolites that are more readily eliminated.

Tissue retention and organ sensitivity jointly influence the pattern of disease.

Slide 21 · Possible destinations of an absorbed xenobiotic

Local action

Biologic effect at the entry site

Remote action

Effect after vascular transport

Storage

Retention in bone, fat, brain or other tissues

Elimination

Excretion unchanged or after conversion

These are competing or sequential fates—not necessarily mutually exclusive.

Slide 22 · Storage sites cited

Potential reservoir

Pathology interpretation

Bone

A possible compartment for persistent material

Adipose tissue

A relevant reservoir for some lipid-soluble agents

Brain

A site where accumulation may matter to neural function

Other tissues

Storage depends on the agent and tissue chemistry

The source gives illustrative reservoirs, not a fixed destination for every xenobiotic.

Slide 23 · Exit routes of chemicals and metabolites

Chemical / metabolite

Urine, feces or expired air

Removal from body

Some agents are eliminated by renal or gastrointestinal routes.

Other compounds leave the body in expired air.

Biotransformation often increases water solubility and facilitates excretion.

Slide 24 · Persistence and excretion are different endpoints

Retention

Agent remains within an organ or tissue compartment

Toxicity can persist while biologically active material remains

Storage does not guarantee injury; Identity and target matter

Clearance

Unchanged compound or metabolite leaves the body

Water solubility often promotes excretion

Removal can terminate or limit tissue exposure

Compare chemical half-life and tissue vulnerability conceptually; The supplied core excerpt gives no numerical half-lives.

Slide 25 · Integrated xenobiotic disposition

Exposure and uptake

Distribution / storage

Metabolism

Excretion or toxicity

The same absorbed agent may partition into storage, be biotransformed, or reach sensitive organs.

Metabolic conversion is a branching point: Detoxification and activation have opposite consequences.

An exposure history is incomplete without some understanding of disposition.

Slide 26 · Biotransformation can protect or injure

Many xenobiotics are converted to inactive, water-soluble products.

Some metabolic pathways instead generate chemically reactive intermediates.

The original chemical and its metabolites may have different biologic properties.

Therefore, metabolic enzyme activity can either decrease or increase toxicity.

Slide 27 · The two-phase metabolism concept

Parent xenobiotic

Phase I reactions

Phase II conjugation

Elimination

Phase I commonly involves oxidation, reduction or hydrolysis.

Phase II introduces or conjugates groups, often increasing water solubility.

This scheme is a major pattern, not a compulsory sequence for every chemical.

Slide 28 · Phase I: Three reaction classes

Reaction

Core chemical change

Source context

Oxidation

Oxidative chemical transformation

Often CYP-catalyzed

Reduction

Chemical reduction

One form of phase I conversion

Hydrolysis

Bond cleavage through reaction with water

Another form of phase I conversion

Phase I does not guarantee detoxification: The metabolite may be reactive.

Slide 29 · Phase I is not always a safety mechanism

Phase I products may be less harmful, unchanged in hazard, or more reactive.

CYP450 catalyzes many phase I processes.

Formation of a toxic intermediate is called metabolic activation or bioactivation.

This explains why a parent compound can become injurious only after enzyme-mediated conversion.

Slide 30 · Phase II: Conjugation pathways in the source

Reaction

Principle

Glucuronidation

Addition of a glucuronide-containing group

Sulfation

Sulfate conjugation

Methylation

Transfer of a methyl group

Glutathione (GSH) conjugation

Conjugation involving the antioxidant GSH

These reactions commonly favor water-soluble products; Pathways depend on substrate.

Slide 31 · Water solubility helps eliminate xenobiotics

Phase II metabolism commonly yields products that are more water-soluble.

Water-soluble chemicals are more readily eliminated from the body.

Successful detoxification depends on forming a product that lacks harmful reactivity.

A conjugate should not automatically be described as nontoxic without knowing the specific agent.

Slide 32 · Detoxification versus metabolic activation

Detoxification

Parent xenobiotic undergoes metabolic conversion

Nontoxic product forms

Elimination limits ongoing exposure

Activation

Metabolism creates a reactive intermediate

Cellular molecules become targets

Damage may outstrip cellular repair

These opposing branches form the core of the textbook’s xenobiotic-metabolism model.

Slide 33 · Reactive metabolite to tissue injury

Reactive product

Molecular interaction

Cellular damage

Organ dysfunction

Reactive products may affect enzymes, receptors, membranes or DNA.

The type of molecular target influences the pattern of pathology.

Cellular repair may prevent a damaging interaction from becoming overt disease.

Slide 34 · Principal molecular targets

Proteins / enzymes

Disrupted molecular function

Receptors

Altered cellular response

Membranes

Impaired cell-compartment integrity

DNA

Genetic injury and possible long-term sequelae

Robbins explicitly names these cellular components as targets of reactive metabolites.

Slide 35 · Cellular repair determines whether injury persists

Molecular damage does not inevitably cause irreversible tissue injury.

Cells possess mechanisms to repair damaged molecules and structures.

When the burden of damage exceeds repair capacity, toxicity emerges.

Persistent damage can produce short-term or long-term adverse outcomes.

Slide 36 · Injury severity: Dose and duration matter

Low or brief insult

Repair / adaptation

Potential recovery

Robbins Chapter 2 contrasts reversible injury after limited exposure with cell death after more severe or prolonged insults.

Sensitivity also depends on cell type, metabolic state and adaptability.

The diagram is conceptual: No universal exposure boundary exists between reversible and irreversible damage.

Slide 37 · Reversible injury versus cell death

Reversible injury

Lesser or shorter toxic insult

Cell dysfunction may recover after the agent is removed

Repair capacity remains adequate

Irreversible injury

Severe or sustained insult

Essential cell components fail

Cell death and tissue damage may follow

A single agent can cause either outcome at different exposure intensities.

Slide 38 · Toxicants can damage several systems at once

The same stimulus can affect mitochondria, membranes, DNA and protein machinery.

Multiple pathways may interact rather than act independently.

This is why microscopic patterns of toxic injury may not reveal a single causal mechanism.

Interpret organ pathology together with agent exposure and biochemistry.

Slide 39 · Cytochrome P450 is the major phase I catalyst

The CYP450 system is a large family of heme-containing enzymes.

Individual CYP enzymes have preferred, but not exclusive, substrate specificities.

They participate in metabolism of solvents, chemicals, alcohol and numerous drugs.

CYP-mediated conversion may detoxify chemicals or generate injurious metabolites.

Slide 40 · Where CYP enzymes are expressed

Liver

Hepatocytes are a major site of CYP expression

Many enzymes localize to endoplasmic reticulum

Hepatic metabolism shapes systemic chemical disposition

Extrahepatic tissues

Skin, lung and GI mucosa also contain CYP enzymes

Local metabolism can occur near an entry surface

Both detoxification and activation are possible

Tissue location affects where enzymatic products are generated.

Slide 41 · Hepatic and extrahepatic metabolism

Hepatocytes

High CYP expression

Major organ for metabolism of absorbed compounds

Can be a target of CYP-dependent toxic products

Other organs

Skin, lung and gastrointestinal mucosa express CYPs

Metabolism can occur near the portal of exposure

Local activation can make an exposed tissue vulnerable

Do not assume all xenobiotic metabolism is restricted to the liver.

Slide 42 · CYP enzymes are heme-dependent catalysts

The family contains many distinct CYP isoenzymes with varied substrate preferences.

They perform chemical transformations central to phase I handling of xenobiotics.

CYP activity depends on the particular enzyme and the encountered substrate.

Consequently, naming “CYP450” alone is insufficient to predict a chemical’s precise fate.

Slide 43 · CYP catalysis also produces ROS

CYP-mediated oxidation

Reactive oxygen species

Cellular oxidative damage

Reactive oxygen species can arise as by-products of CYP metabolism.

Oxidative injury can accompany either otherwise protective or activating conversions.

ROS to broader mechanisms of cell injury discussed in Chapter 2.

Slide 44 · The CYP paradox: Protection and bioactivation

Metabolism commonly terminates activity and helps excrete chemicals.

Less commonly, CYP conversion increases chemical reactivity and toxic potential.

The damaging agent may therefore be a metabolite rather than the environmental parent.

Mechanistic attribution should identify the reactive species when established.

Slide 45 · Why CYP activity varies across individuals

Genetic polymorphisms in specific CYP genes produce one source of variation.

Environmental chemical exposures and medications can modify CYP abundance or activity.

Smoking, alcohol and hormones are among factors associated with induction.

Fasting and starvation may decrease CYP activity.

Slide 46 · Modifiers of CYP activity in the Robbins text

Genotype

Polymorphisms in specific CYP genes

Chemical exposure

Environmental substances and medications

Behavior / physiology

Smoking, alcohol, hormonal influences

Nutritional state

Fasting or starvation may reduce activity

Consequences are substrate-dependent: Altered activity may increase clearance or bioactivation.

Slide 47 · CYP induction is transcriptionally regulated

Inducing ligand

Nuclear receptor signaling

CYP gene transcription

Enzyme activity

Inducing chemicals bind regulatory receptors described in the source.

Activated receptors help recruit transcriptional machinery to CYP regulatory regions.

The impact on toxicity depends on which CYP and metabolic route are affected.

Slide 48 · Nuclear receptors regulate CYP transcription

Chemical exposure can activate nuclear-receptor-mediated transcription.

Receptor partners, including retinoid X receptor (RXR), in this regulatory network.

Transcriptional complexes bind CYP gene regulatory sequences.

Different receptor systems should not be assumed to share identical binding partners or transcriptional mechanisms.

Slide 49 · Receptors named in xenobiotic enzyme regulation

Source-listed regulator

Abbreviation

Aryl hydrocarbon receptor

AhR

Peroxisome proliferator-activated receptors

PPARs

Constitutive androstane receptor

CAR

Pregnane X receptor

PXR

These regulators link chemical exposure to xenobiotic-metabolizing gene expression.

Slide 50 · Enzyme induction has two possible outcomes

When a metabolite is harmless

Greater conversion can enhance elimination

Parent-compound exposure may decrease

Detoxifying pathway can dominate

When a metabolite is reactive

Greater conversion can increase activation

Cellular injury may become more likely

Total outcome also depends on competing pathways

Induction is not synonymous with protection or toxicity; Identify the substrate and product.

Slide 51 · Illustrative CYP substrates

Therapeutic agents

Acetaminophen

Barbiturates

Warfarin and anticonvulsants

Other exposures

Alcohol

Carbon tetrachloride

Benzo[a]pyrene in cigarette smoke

Different substrates need not share the same CYP enzyme or injury pattern.

Slide 52 · Carbon tetrachloride: Bioactivation example

Carbon tetrachloride

Hepatic CYP metabolism

Trichloromethyl free radical

Cellular damage

CCl₄ to illustrate generation of a toxic radical by hepatic metabolism.

The parent xenobiotic becomes hazardous through a reactive intermediate.

This is a classic mechanistic contrast with phase II-mediated detoxification.

Slide 53 · Benzo[a]pyrene: A genotoxic activation example

Cigarette-smoke compound

CYP conversion

DNA-binding metabolite

Genotoxic injury

Benzo[a]pyrene is an example of a procarcinogen that requires metabolic activation.

The activated product can bind DNA in lung tissue.

Genotoxic effects illustrate a potential long-term consequence of xenobiotic metabolism.

Slide 54 · Acetaminophen: Competing metabolic fates

Predominant conjugation pathway

Glucuronidation and sulfation

Products eliminated in urine

Detoxification predominates at therapeutic exposures

Minor CYP-dependent pathway

CYP metabolism generates NAPQI

NAPQI is reactive and potentially hepatotoxic

Normal GSH conjugation limits exposure

These are parallel metabolic fates; The proportion entering reactive pathways matters.

Slide 55 · GSH determines the outcome of NAPQI exposure

GSH is sufficient

Reactive NAPQI is conjugated

Inactivation reduces chemical reactivity

Hepatocyte injury is limited

GSH becomes depleted

Unconjugated NAPQI accumulates

Protein binding and mitochondrial dysfunction

Oxidant injury and centrilobular necrosis

The critical fork is reactive metabolite formation relative to available detoxification.

Slide 56 · Acetaminophen: Morphology follows chemistry

The Robbins example links excessive NAPQI to centrilobular hepatocellular necrosis.

Severe injury can progress from hepatocyte damage to acute liver failure.

The named molecular mechanisms include protein binding and GSH depletion.

This is an application of the general parent-compound → metabolite → target-organ sequence.

Slide 57 · Ozone and carbon monoxide: Two toxicity patterns

Ground-level ozone

Oxidant exposure at the respiratory surface

ROS-associated epithelial damage

Airway inflammation and reduced lung function

Carbon monoxide

Absorbed systemically after inhalation

Formation of carboxyhemoglobin

Impaired oxygen transport and tissue hypoxia

Adjacent chapter examples contrast local chemical oxidant injury with systemic oxygen deprivation.

Slide 58 · Physical agents: Interpret the kind of energy

Robbins explicitly extends toxicology to radiation and heat.

The later chapter separates mechanical, thermal, electrical and ionizing-radiation injury.

Unlike xenobiotic metabolism, the primary exposure variable may be deposited energy or temperature disturbance.

Each physical-agent category has separate mechanisms and is addressed in its own indexed unit.

Slide 59 · Chemical versus physical injury: Conceptual comparison

Question

Chemical agents

Physical agents

Primary exposure

Compound or reactive metabolite

Energy, force or thermal stress

Internal disposition

Absorption, distribution and metabolism

Energy transfer / physical tissue effect

Important modifier

Bioactivation, retention, host enzymes

Magnitude, duration, tissue vulnerability

Shared final outcome

Cellular dysfunction, injury or death

Cellular dysfunction, injury or death

Conceptual overview, not a diagnostic classification.

Slide 60 · Case study: Inhaled compound with distant effects

HYPOTHETICAL TEACHING SCENARIO

A worker inhales a chemical vapor and later develops dysfunction in an organ other than the lung.

What two steps are needed to explain a remote target-organ effect?

Pulmonary absorption followed by vascular distribution.

Chemical transformation may additionally create an active metabolite, but remote injury need not require bioactivation.

Slide 61 · Case study: Identical exposure, different response

HYPOTHETICAL TEACHING SCENARIO

Two individuals encounter the same nominal exposure; Only one develops clinically evident injury.

Which host-related factors could explain this difference?

Differences in age, genetics, tissue sensitivity, concurrent exposure, or metabolic enzyme activity.

Exposure equivalence does not prove equivalent internal dose or organ-level sensitivity.

Slide 62 · Case study: The parent molecule appears inert

HYPOTHETICAL TEACHING SCENARIO

A chemical becomes hazardous only after conversion by hepatic enzymes.

Which metabolic process explains the new toxicity?

Bioactivation, commonly through a phase I/CYP-catalyzed pathway.

The reactive metabolite—not necessarily the original molecule—may interact with membranes, proteins or DNA.

Slide 63 · Case study: Excessive analgesic exposure

HYPOTHETICAL TEACHING SCENARIO

A hypothetical patient with excessive acetaminophen ingestion develops hepatocellular necrosis.

Which competing processes explain the injury?

CYP-mediated NAPQI formation, inadequate GSH conjugation, and reactive metabolite injury.

The later Robbins drug section identifies these processes and the centrilobular morphology.

Slide 64 · Synthesis: How to reason through any toxic exposure

Exposure / route

Internal dose / fate

Reactive target / repair

Clinical injury

Specify the agent and exposure history before interpreting the pathology.

Identify whether clearance, storage or enzyme-dependent activation dominates.

Integrate the affected molecular target, tissue susceptibility and observed injury.

Slide 65 · Five principles to retain

Dose and exposure pattern determine whether a chemical behaves as a poison.

Xenobiotics enter through lung, GI tract or skin and may act locally or systemically.

Metabolism can detoxify or activate; Phase I and phase II are not universally protective steps.

CYP450 enzyme variability and receptor-mediated regulation modify susceptibility.

Cellular repair, tissue vulnerabilities and time determine whether injury resolves or persists.

Slide 66 · Core source and contextual readings

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. 2026. Chapter 9, “Toxicity of Chemical and Physical Agents,” pp. 371–372.

Chapter 9, Fig. 9.3: exposure to pollutants; Fig. 9.4: xenobiotic metabolism.

Related applied examples in Chapter 9: outdoor air pollution, acetaminophen injury, and injury by physical agents.

General cellular injury context: Chapter 2, “Mechanisms of Cell Injury.”