Open with the Flint, Michigan water crisis. Tell students: in April 2014 Flint switched its water source to the corrosive Flint River without adding orthophosphate corrosion inhibitor. Within 18 months the percentage of Flint children under 5 with blood lead levels ≥5 µg/dL nearly doubled (from ~2.4% to ~4.9%, and up to ~10% in high-risk zones — Hanna-Attisha et al., 2016). Ask: 'The lead did not come from paint. Where did it come from, and why did small children absorb more of it than their parents?' Take 2–3 quick answers, do not resolve — tell students today they will trace the pathway and interpret the epidemiology themselves. Targets SP 3 and SP 7.

1. 8m

   Toxicology Foundations: Dose, Duration, and Type of Harm

   Content

   Toxicology rests on Paracelsus's principle — 'the dose makes the poison.' Every pollutant has a dose-response relationship: as dose increases, the fraction of a population showing an effect increases, often described by LD₅₀ (the dose lethal to 50% of test organisms). Two exposure regimes matter. Acute exposure is a single high dose over minutes to hours — e.g., a worker inhaling a chlorine cloud, with immediate pulmonary edema. Chronic exposure is repeated low doses over months to years — e.g., a child drinking 0.015 mg/L Pb tap water daily for two years, producing IQ loss and attention deficits with no acute symptoms. Pollutants are also classified by the type of biological harm: neurotoxins (Pb, methylmercury, organophosphates) disrupt nervous system signaling; carcinogens (benzene, asbestos, arsenic, formaldehyde) cause cancer, usually via chronic low-dose exposure with latency of 10–40 years; teratogens (methylmercury, thalidomide, ethanol) disrupt fetal development. A key AP idea: the dose-response curve for a chronic carcinogen has no clear 'safe' threshold — regulators use linear no-threshold models, which is why EPA sets the Pb action level for drinking water at 0.015 mg/L and the goal at 0 mg/L.

   Delivery

   Emphasize that most students walk in thinking pollution = acute poisoning like a spill. Push them on the timescale: 'If a child's IQ drops 5 points by age 6, when did the exposure happen?' Cue up the anchor visual — a dose vs. severity graph with two curves (acute high-dose spike, chronic low-dose plateau) — and narrate how a chronic curve can produce widespread harm at doses no individual would notice. Ask cold: 'Why does EPA set the Pb goal at zero?' Correct the misconception that 'low dose = safe' — for carcinogens and developmental neurotoxins, there may be no safe threshold. Targets SP 2 and SP 5.

2. 7m

   Exposure Pathways: How Contaminants Reach the Body

   Content

   An exposure pathway has five links: (1) source, (2) environmental medium (water, air, soil, food), (3) point of contact, (4) route of entry, (5) receptor population. A contaminant is only a hazard when all five connect. Three routes of entry matter: ingestion (drinking Pb-contaminated water, eating Hg-contaminated tuna, hand-to-mouth transfer of Pb dust by toddlers), inhalation (PM₂.₅ from wildfire smoke, asbestos fibers, Hg vapor from a broken thermometer or artisanal gold mining), and dermal absorption (organic solvents, some pesticides, and — importantly — most heavy metal ions do NOT cross intact skin well, an AP nuance). For lead specifically, sources are not just paint: corroded Pb service lines and Pb solder in plumbing, Pb dust from deteriorating paint tracked into soil, contaminated soil near old smelters or highways (leaded gasoline residue), and imported spices/cosmetics. A child crawling on a floor, then putting hands in mouth, converts soil-Pb into ingested-Pb — the same source becomes a different pathway depending on behavior.

   Delivery

   Walk students through the pathway diagram on the slide as a five-box chain and stress that breaking ANY link stops exposure — this frames the SP 7 solutions work later. Directly confront the paint-only misconception with the Flint case: 'The Pb in Flint water came from service lines corroded by untreated river water. No paint involved.' Then confront the ingestion-only misconception: 'Miners inhaling Hg vapor absorb ~80% of it into blood. Skin? Almost none for ionic Hg²⁺.' Ask: 'For a 2-year-old versus a 30-year-old exposed to the same soil, which pathway dominates and why?' (Hand-to-mouth behavior + higher soil intake per kg body mass.) Targets SP 2 and SP 7.

3. 6m

   Vulnerable Populations and Biomagnification: The Mercury Case

   Content

   Two children and one adult exposed to identical water Pb concentrations do NOT receive identical doses or suffer identical harm. Reasons: (a) children drink more water and eat more food per kg body mass — roughly 2–3× the adult rate; (b) their gastrointestinal absorption of Pb is ~40–50% versus ~5–10% in adults; (c) the blood-brain barrier is not fully developed until ~age 2; (d) their nervous systems are still wiring, so a neurotoxin during a developmental window causes permanent deficits. Fetuses are the most vulnerable — the placenta does NOT block Pb or methylmercury. Mercury illustrates a second amplifier: biomagnification. Coal combustion and artisanal gold mining release inorganic Hg to the atmosphere; it deposits into water, where anaerobic bacteria methylate it to CH₃Hg⁺, which is lipid-soluble and binds protein. Concentrations multiply roughly 10× per trophic level, so a top predator (swordfish, shark, tuna) can carry Hg 10⁵–10⁶× the water concentration. The Minamata, Japan disaster (1956) — Chisso Corporation dumped methylmercury into Minamata Bay; residents eating local fish suffered ataxia, sensory loss, and severe birth defects (congenital Minamata disease) even when mothers were asymptomatic. This is why FDA advises pregnant women to avoid swordfish and limit albacore tuna.

   Delivery

   Anchor this beat with the pyramid-shaped biomagnification visual (Hg concentration by trophic level). Ask: 'Why can a mother appear healthy while her fetus is severely harmed?' — the answer (placental transfer + developmental window) directly targets the misconception that adults and children are equally vulnerable. Then ask the SP 7 stinger: 'If you regulate Hg, do you regulate the water or the coal plants?' Push them toward source reduction. Targets SP 2 and SP 5.

1. 30m

   Case Study: Flint Pathway Map + Epidemiological Data Analysis

   Students work in pairs for 25 minutes, then 5-minute whole-class debrief. Part 1 (10 min) builds the pathway diagram; Part 2 (10 min) is data analysis; Part 3 (5 min) is an evidence-based recommendation. Circulate and check pathway arrows and unit work. Targets SP 2 (Visual Representations), SP 5 (Data Analysis), SP 6 (Mathematical Routines), and SP 7 (Environmental Solutions). Student handout: Case Study — Flint, Michigan (2014–2016) In April 2014 the city of Flint switched its drinking-water source from Lake Huron (treated by Detroit) to the Flint River. Flint River water was ~19× more corrosive to iron than Lake Huron water, and the treatment plant did NOT add an orthophosphate corrosion inhibitor. Roughly half of Flint's service lines were lead. Data below are adapted from Hanna-Attisha et al. (2016) and Edwards et al. (Virginia Tech). Part 1 — Build the Exposure Pathway (10 min) Using three colors, draw a 5-box pathway diagram on the space provided for a 2-year-old child living in a Flint home with a Pb service line. Label each box: 1. Source = ______ 2. Environmental medium = ______ 3. Point of contact = ______ 4. Route of entry = ______ 5. Receptor = ______ Then, in one or two sentences, explain the chemistry link: why did switching to Flint River water cause Pb to leach from pipes that had been in the ground for decades without a problem? Part 2 — Epidemiological Data (10 min) Data table: percent of children under 5 with blood Pb ≥5 µg/dL (CDC reference level) - Zone A (high water Pb, >15 µg/L): before switch 4.0%, after switch 10.6% - Zone B (moderate water Pb, 5–15 µg/L): before switch 2.0%, after switch 4.0% - Zone C (low water Pb, <5 µg/L): before switch 0.8%, after switch 1.2% - Comparison city (Detroit, unchanged source): before 2.5%, after 2.4% Answer: a) Calculate the ratio (after ÷ before) for each Flint zone and for Detroit. Show work. - Zone A: ______ - Zone B: ______ - Zone C: ______ - Detroit: ______ b) Which pattern in your ratios provides the strongest evidence that water Pb — not paint, soil, or another confounder — caused the increase? Explain in 2–3 sentences using the term dose-response. c) A skeptic claims: 'The increase could be from lead paint in old Flint housing.' Using the Detroit comparison and the zone gradient, refute this claim in 2 sentences. d) A pediatrician measures a 14-month-old's blood Pb at 8 µg/dL. There is no acute illness. Is this exposure acute or chronic? What health outcomes should the family expect over the next 10 years? Part 3 — Propose a Solution (5 min) The pathway has 5 links. For each link, propose ONE intervention that would break it. Rank your five interventions from fastest-acting (days) to most permanent (decades) and justify the ranking in one sentence. - Break link 1 (source): ______ - Break link 2 (medium): ______ - Break link 3 (point of contact): ______ - Break link 4 (route): ______ - Break link 5 (receptor protection): ______ Do not skip Part 3 — this is the SP 7 target and will appear on the exit ticket.

   Materials

   • Printed student handout (one per pair)
   • Colored pencils or pens (3 colors per pair)
   • Calculator
   • Optional: computer to view EPA lead pathway simulation

   Example outputs

   • Part 1 pathway: Source = Pb service line & Pb solder → Medium = tap water (corrosive Flint River water without orthophosphate) → Point of contact = kitchen faucet used for formula/drinking → Route = ingestion → Receptor = children under 5 in homes with Pb lines. Chemistry: corrosive water without a phosphate coating dissolves the passivating scale on the Pb pipe interior, releasing soluble Pb²⁺ and Pb particulates into tap water.
   • Part 2a ratios: Zone A 10.6/4.0 = 2.65×; Zone B 4.0/2.0 = 2.0×; Zone C 1.2/0.8 = 1.5×; Detroit 2.4/2.5 = 0.96×. 2b: The dose-response gradient — higher water Pb zones show larger increases while Detroit (unchanged source) is flat — is causally consistent with water as the exposure source. 2c: Detroit housing is similarly old and also has Pb paint, yet Detroit's rate did not change; and within Flint the increase tracks water Pb, not housing age uniformly. 2d: Chronic; expect IQ deficits (~4–7 points), attention/behavior problems, and possible reduced academic performance.
   • Part 3 example: (1) Source — replace Pb service lines (permanent, years); (2) Medium — add orthophosphate corrosion inhibitor (days–weeks, fastest); (3) Point of contact — install certified NSF-53 Pb-removing filter on kitchen tap; (4) Route — use bottled water for drinking/cooking (immediate but not durable); (5) Receptor — screen and chelate children with BLL >45 µg/dL, nutrition counseling to reduce Pb absorption. Ranked fastest→most permanent: orthophosphate → bottled water → filter → chelation → line replacement.
   • Presentation_text example for slide

1. A coal-fired power plant emits inorganic mercury to the atmosphere. Anaerobic bacteria in a downstream lake convert it to methylmercury. Which population is at greatest risk of neurological harm, and why? (A) Adult sport fishers eating one bass per month, because they have the greatest cumulative exposure. (B) Fetuses of pregnant women who regularly eat top-predator fish, because methylmercury crosses the placenta during nervous-system development. (C) Swimmers in the lake, because dermal absorption of Hg²⁺ is rapid. (D) Children drinking municipal tap water, because Hg biomagnifies in drinking water. Targets SP 2 and SP 7.

   multiple choiceB. Methylmercury is lipid-soluble, crosses the placenta, and disrupts the developing fetal nervous system during a critical window; biomagnification concentrates it in top-predator fish. A underestimates the fetal window; C is wrong because ionic Hg is poorly absorbed through skin; D is wrong because biomagnification happens in the food web, not drinking water.

2. A community lives 500 m downwind of a former Pb smelter closed in 1985. Soil Pb near homes averages 800 mg/kg (EPA residential screening level = 200 mg/kg). Children ages 1–3 have a mean blood Pb of 6.2 µg/dL; adults in the same homes average 1.4 µg/dL. In 3–4 sentences, describe the dominant exposure pathway for the toddlers AND explain why children show ~4× higher blood Pb than adults from the same environment. Targets SP 2.

   short answerPathway: Pb from historic smelter emissions deposited in surface soil (source → soil medium) → toddlers contact soil during outdoor play and floors indoors where soil is tracked in (point of contact) → hand-to-mouth behavior transfers Pb-dust to ingestion route → receptor is the toddler. Children show higher BLLs because (1) hand-to-mouth behavior gives them a much larger soil intake per kg body mass, (2) GI absorption of Pb is ~40–50% in young children vs. ~5–10% in adults, and (3) the immature blood-brain barrier and developing CNS mean the same absorbed dose has greater physiological effect.

3. A drinking-water utility must decide between two interventions to reduce childhood Pb exposure: (A) Full replacement of all Pb service lines over 15 years at $500 million, or (B) Immediate addition of orthophosphate corrosion inhibitor at $2 million/year. Using the concept of exposure-pathway links and the distinction between acute and chronic risk, argue which option should be implemented FIRST and justify why the other should still follow. Targets SP 7.

   short answerImplement B first: orthophosphate coats pipe interiors within weeks, breaking the 'medium' link (Pb²⁺ dissolution into water) at low cost and reducing chronic low-dose exposure across the entire service area almost immediately — critical because Pb harm to children is cumulative and every month of exposure adds IQ loss. However, B is not permanent — a future treatment lapse (as in Flint) would restore the hazard, so A must follow: replacing service lines eliminates the source itself and provides durable protection. The two interventions target different pathway links (source vs. medium) and both are justified; sequencing prioritizes the fastest reduction of chronic dose.

4. Which pairing of pollutant → primary health effect → most sensitive exposure route is CORRECT? (A) Asbestos → mesothelioma → dermal absorption. (B) Methylmercury → liver cancer → inhalation. (C) PM₂.₅ → cardiovascular and respiratory disease → inhalation. (D) Lead → skin rash → ingestion of contaminated soil. Targets SP 1.

   multiple choiceC. PM₂.₅ particles ≤2.5 µm penetrate deep into alveoli, enter the bloodstream, and drive cardiovascular and respiratory disease via inhalation. A is wrong (asbestos causes mesothelioma via INHALATION of fibers, not skin); B is wrong (methylmercury is a neurotoxin/teratogen ingested in fish, not a liver carcinogen inhaled); D is wrong (Pb causes neurological damage, not skin rash).

• ☐Printed Flint case-study handout (one per pair, ~2 pages)
• ☐Colored pencils or pens — at least 3 colors per pair
• ☐Basic calculator per pair
• ☐Projector for slide deck (dose-response, pathway, biomagnification pyramid visuals auto-load)
• ☐Exit-ticket copies of the 4 formative-assessment items
• ☐Optional: computer access to EPA Pb information hub (epa.gov/lead)