Environmental Pollution

• Primary vs secondary air pollutants with sources and effects
• Photochemical smog (PAN) and London smog — formation and distinction
• Water quality measurement using BOD; heavy metal pollution diseases
• Eutrophication mechanism and consequences for aquatic ecosystems
• Soil pollutants — DDT bioaccumulation, biomagnification, and plastic waste
• Noise pollution thresholds and health effects

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Air Pollution — Primary Pollutants

Primary pollutants are directly emitted from sources.

Tetraethyl lead was used as anti-knock agent. Now replaced by unleaded petrol + catalytic converters. Still a problem in countries with old vehicle fleets.

CO mechanism of toxicity:

Hb + O₂ ⇌ HbO₂ (oxyhaemoglobin) — normal
Hb + CO → HbCO (carboxyhaemoglobin) — IRREVERSIBLE under normal conditions
Affinity of Hb for CO is ~240× greater than for O₂

Even 0.1% CO in air → 50% HbCO → unconsciousness and death.

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Air Pollution — Secondary Pollutants

Secondary pollutants form in the atmosphere by chemical reactions of primary pollutants.

Photochemical Smog (Los Angeles Type)

Conditions: sunny, warm, traffic-heavy cities; summer afternoons

Step 1: NO₂ + hν (UV) → NO• + O•        (photodissociation)
Step 2: O• + O₂ → O₃                   (tropospheric ozone formed — BAD)
Step 3: O₃ + NO → NO₂ + O₂             (NO₂ regenerated — cycle)
Step 4: VOC + NO₂ + O₂ → PAN + other products

PAN (Peroxyacetyl Nitrate): CH₃CO–O–O–NO₂

• Severe eye irritant, causes tears/burning
• Damages photosynthetic tissue of plants (bleaching)
• Formed from reaction of NOₓ with hydrocarbons/aldehydes in sunlight

London Smog (Classical Smog)

Conditions: cold, foggy winter mornings; coal-burning areas

SO₂ + fog + particulates + O₂ → H₂SO₄ aerosol (acidic, reducing smog)

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Water Pollution

BOD (Biochemical Oxygen Demand)

BOD = amount of dissolved O₂ (in ppm) consumed by microorganisms to decompose organic matter in 1 L of water sample at 20°C over 5 days.

Higher organic pollution → more microbes → more O₂ consumed → higher BOD → less O₂ for fish → aquatic life suffers.

Heavy Metal Pollution

Methylmercury (CH₃Hg⁺) is the most toxic form — produced by anaerobic bacteria from inorganic Hg in sediments. Bioaccumulates in fish → biomagnifies up food chain.

Eutrophication

Eutrophication = excessive enrichment of water body with nutrients (mainly N and P from agricultural runoff and sewage).

Mechanism:
1. Excess nitrates/phosphates → rapid algal growth (algal bloom) — green surface layer
2. Algae die → bacterial decomposition → bacteria consume O₂
3. Dissolved O₂ depleted (hypoxia/anoxia) → fish and other aquatic life die
4. "Dead zones" form (e.g., Gulf of Mexico, Baltic Sea)

Phosphates (from detergents, fertilisers) are the limiting nutrient in freshwater systems — even small amounts trigger bloom.

Other Water Pollutants

• Fluoride (>1.5 ppm): skeletal fluorosis; deficiency also harmful (dental decay)
• Nitrates (>45 ppm in drinking water): methaemoglobinaemia (blue baby syndrome) — converts HbFe²⁺ to HbFe³⁺ (cannot carry O₂)

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Soil Pollution

Pesticides — DDT Bioaccumulation

DDT (Dichlorodiphenyltrichloroethane): Organochlorine pesticide; very effective against malarial mosquitoes.

Problem — Biomagnification:

Soil/water: DDT conc ~ 0.003 ppm (trace)
      ↓  absorbed by plankton
Plankton:   0.04 ppm (×13)
      ↓
Small fish: 0.5 ppm (×12.5)
      ↓
Large fish: 2 ppm (×4)
      ↓
Fish-eating birds (osprey/bald eagle): 25 ppm (×12.5)

Biomagnification = progressive increase in concentration of non-biodegradable pollutant at each trophic level.

DDT effects:

• Eggshell thinning in birds (inhibits Ca²⁺ enzyme) → reproductive failure
• Endocrine disruptor in mammals
• Banned in many countries but persists in environment for decades

Other Soil Pollutants

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Noise Pollution

• >85 dB: prolonged exposure → hearing loss (noise-induced hearing loss, NIHL)
• >100 dB: acute pain, immediate cochlear damage
• Permissible limit (OSHA/India): 85 dB for 8-hour workday
• Effects beyond hearing: hypertension, stress, sleep disturbance, reduced productivity

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PAN Formation — Detailed Chemistry

Acetaldehyde (CH₃CHO) from incomplete combustion:
CH₃CHO + OH• → CH₃CO• + H₂O      (initiation by OH radical)
CH₃CO• + O₂ → CH₃C(O)OO•         (peroxyacetyl radical)
CH₃C(O)OO• + NO₂ → CH₃C(O)OONO₂  (PAN formed)

PAN = peroxyacetyl nitrate; unstable above 40°C (decomposes)
→ acts as "reservoir" for NOₓ transport in cold upper atmosphere

CO Toxicity — Quantitative

At 100 ppm CO in air (workplace limit), ~15% HbCO → headache At 400 ppm → 25% HbCO → severe headache, dizziness At 1000 ppm → 65% HbCO → death within 1 hour

Methylmercury Biomagnification

Inorganic Hg²⁺ → methylated by anaerobic bacteria (Desulfovibrio) in sediment → CH₃Hg⁺ (methylmercury). This is lipophilic → partitions into fatty tissue → not excreted → concentration amplified at each trophic level (biomagnification factor ~10⁶ from water to top predator).

Nitrate → Methaemoglobinaemia

NO₃⁻ (ingested) → reduced by gut bacteria → NO₂⁻
NO₂⁻ + HbFe²⁺ → HbFe³⁺ (methaemoglobin) + NO₃⁻

HbFe³⁺ cannot bind O₂ → cyanosis (blue colour)
Infants most vulnerable (lower gastric pH, gut bacteria more reductive)

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Example 1: BOD analysis

Problem: Sample A has BOD = 3 ppm; Sample B has BOD = 15 ppm. Compare
the two samples and predict which would be safe for aquatic life.

Step 1: Define BOD
  BOD = dissolved O₂ consumed by microbes to decompose organic matter
  in 1 L water over 5 days at 20°C.

Step 2: Interpret values
  Sample A: BOD = 3 ppm < 5 ppm → fairly clean water
    → low organic load → microbes consume little O₂
    → sufficient dissolved O₂ remains for aquatic life
  
  Sample B: BOD = 15 ppm > 10 ppm → highly polluted water
    → high organic load → intense microbial activity
    → most dissolved O₂ consumed
    → DO may fall below 4–5 ppm → fish and aerobic organisms die

Step 3: Conclusion
  Sample A is safe for aquatic life (clean, BOD < 5 ppm)
  Sample B is highly polluted (BOD > 10 ppm → oxygen depletion → aquatic death)

Note: Normal saturated dissolved O₂ in water at 20°C ≈ 9 ppm.
If BOD = 15 ppm > 9 ppm available O₂ → complete anoxia is possible.

Example 2: Biomagnification of DDT

Problem: If DDT concentration in water is 0.003 ppm and undergoes
10-fold biomagnification at each trophic level, calculate the concentration
in (a) plankton, (b) small fish eating plankton, (c) large fish,
(d) osprey eating large fish. State the environmental consequence.

Step 1: Biomagnification factor = 10× per trophic level

Step 2: Calculations
  Water:       0.003 ppm  (base)
  Plankton:    0.003 × 10 = 0.03 ppm
  Small fish:  0.03 × 10  = 0.3 ppm
  Large fish:  0.3 × 10   = 3 ppm
  Osprey:      3 × 10     = 30 ppm

Step 3: Environmental consequence
  Osprey DDT concentration: 30 ppm (10,000× water concentration)
  
  At this concentration, DDT inhibits Ca²⁺-ATPase enzyme in oviducts
  → Thin, fragile eggshells produced
  → Eggs crushed during incubation → reproductive failure
  → Population collapse (documented in bald eagles, ospreys, peregrine falcons in USA 1950s–70s)
  
  This led to DDT ban in USA (1972) by EPA.

Answer: Osprey concentration = 30 ppm; consequence = eggshell thinning,
reproductive failure, and population collapse of top predator birds.

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Q1. The Minamata disease is caused by pollution of water with:

A) Cadmium compounds
B) Lead compounds
C) Mercury compounds
D) Arsenic compounds

Q2. BOD of a water sample is 3 ppm. This water is:

A) Heavily polluted and unfit for aquatic life
B) Moderately polluted
C) Fairly clean and safe for aquatic life
D) Completely pure and unfit for biological treatment

Q3. PAN (Peroxyacetyl Nitrate) is a major component of:

A) London smog
B) Photochemical smog
C) Industrial smog from coal burning
D) Acid rain

Q4. Which of the following is the correct sequence of biomagnification of DDT?

A) Plankton < Water < Fish < Birds
B) Water < Plankton < Fish < Birds
C) Birds < Fish < Plankton < Water
D) Water < Fish < Plankton < Birds

Q5. Itai-itai disease is caused by contamination of water with:

A) Mercury
B) Lead
C) Arsenic
D) Cadmium

Answer Key: 1-C | 2-C | 3-B | 4-B | 5-D

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• NCERT Class 12 Chemistry — Chapter 14: Environmental Chemistry
• Pages 408–430 (pollution)
• Key sections: 14.2 Atmospheric pollution, 14.3 Water pollution, 14.4 Soil pollution
• Table 14.2: Diseases caused by heavy metals
• Figure 14.2: Biomagnification of DDT
• Cross-reference: Class 12 Biology Chapter 16 (Environmental Issues) for ecological context

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🎯 Exam Tips

• Disease-metal pairs are high-yield: Minamata = Hg; Itai-itai = Cd — never mix these up
• BOD < 5 ppm = clean water is a direct recall MCQ; remember "5 is the threshold"
• PAN = photochemical smog (not London smog) — this distinction is tested every few years
• Biomagnification direction: concentration INCREASES up the food chain (water → predator)

📊 Weightage

• Environmental Chemistry: 1–2 questions per JEE Main paper
• Pollution sub-topic covers heavy metals, BOD, smog — highest MCQ density in chapter
• Disease-pollution associations appear in both JEE Main and Advanced

🔗 Related Topics

• Atmospheric Chemistry (smog, acid rain — linked to this chapter)
• Biomolecules (haemoglobin structure — CO/NOₓ toxicity)
• Redox Chemistry (BOD measurement involves O₂ consumption = reduction)
• Organic Chemistry (DDT structure, PAN structure)

📝 Revision Checklist

• List 5 primary air pollutants with sources and effects
• Write mechanism of CO toxicity (Hb affinity ratio)
• Distinguish photochemical smog from London smog (5 parameters)
• Define BOD and state the clean/polluted thresholds
• Match heavy metals to diseases: Hg, Cd, As, Pb, F
• Explain eutrophication step-by-step
• Explain DDT biomagnification with example concentrations
• State noise pollution threshold for hearing damage (>85 dB)