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PSM Exam Answers

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Question 1: A study is being designed to determine the effects of radiation among workers at a plant over 5 years.

A) What would be the 'ideal' study design?

• The exposure (radiation) is known and identifiable before disease onset
• The cohort can be followed forward in time to observe who develops disease
• It allows calculation of incidence rates and relative risk
• It is especially appropriate when "exposure is rare but the incidence of disease is high among the exposed, e.g., special exposure groups like those in industries, exposure to X-rays" (Park's Textbook of PSM)
• A prospective design is ideal here because all workers can be enrolled at baseline and followed for 5 years

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B) Steps involved in conducting the cohort study

• Study cohort: Workers with confirmed occupational radiation exposure at the plant
• Control cohort: Workers in the same plant with no or minimal radiation exposure (or a comparable group from a non-radiation industry)
• Both cohorts must be free from the disease under investigation at the time of enrollment
• Both groups must be comparable for all confounding variables (age, sex, smoking, etc.)

• Medical/employment records (dose of radiation received)
• Environmental surveys (radiation levels at work sites)
• Personal interviews or questionnaires (work history, duration)
• Medical examination / special tests (baseline haematological parameters, thyroid function, etc.)
• Classify cohort members by whether and to what degree they were exposed

• Follow both cohorts for the defined period (5 years in this case)
• Track both groups under identical conditions
• Monitor for the outcomes: e.g., cancers (leukemia, thyroid, lung), radiation sickness, chromosomal abnormalities, cataracts, infertility, death

• Calculate disease incidence rate in exposed vs. unexposed groups
• Calculate Relative Risk (RR) = Incidence in exposed / Incidence in unexposed
• Calculate Attributable Risk = Incidence in exposed - Incidence in unexposed

• RR = [a/(a+b)] / [c/(c+d)]

• Regular contact with workers
• Maintain records; use employment registers, death records

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C) Possible disadvantages of a cohort study

1. Time-consuming: For chronic effects of radiation (e.g., cancer), the follow-up may span many years or decades
2. Expensive: Requires sustained resources for 5+ years of follow-up, investigations, and administration
3. Losses to follow-up: Workers may leave the plant, retire, migrate, or die from unrelated causes - this can introduce attrition bias
4. Changes in exposure: Over time, radiation safety protocols may improve, changing exposure patterns mid-study
5. Not suitable for rare diseases: If the expected disease is rare, a very large sample is needed
6. Cohort effect: The study cohort may not be representative of all exposed workers (healthy worker effect - workers are generally healthier than the general population)
7. Confounding: Other factors (smoking, chemical exposure) may affect disease risk and are difficult to control completely
8. Ethical considerations: Continuous follow-up of a population with a known hazard raises ethical issues

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Question 2: Epidemiological determinants of Dengue Fever; Clinical management of DHF; Integrated approach

Epidemiological Determinants of Dengue Fever

• 4 serotypes - infection with one serotype does not protect against others
• Secondary infection with a different serotype increases risk of DHF/DSS (immune enhancement theory)
• Viremia present during febrile phase; virus detected in blood days 1-6

• Age: All ages affected; children at higher risk of DHF in endemic areas
• Sex: No strong predilection, but females may be at higher risk of severe disease in some studies
• Immunity: Primary infection with one serotype gives lifelong immunity to that serotype only; secondary infection is more dangerous
• Nutritional status: Malnourished children may tolerate plasma leakage worse

• Urban/peri-urban areas - dense housing, inadequate water storage, stagnant water collections (flower vases, tires, coolers)
• Season: Transmission peaks during and after monsoon season (Aug-Oct in India)
• Urbanization and globalization allow rapid spread
• Temperature: Aedes mosquito breeding enhanced at higher temperatures (25-30°C)
• Transmission in tropical and subtropical regions (30°N to 30°S latitude)

• Aedes aegypti: day-biting, domestic, breeds in clean stagnant water in man-made containers
• Aedes albopictus: secondary vector, more widespread

• Probable case: Acute febrile illness + 2 or more of headache, retro-orbital pain, myalgia, arthralgia, rash, haemorrhagic manifestations, leucopenia, thrombocytopenia
• Confirmed case: Virus isolation, fourfold rise in IgG, positive RT-PCR, or NS1 antigen detection (Park's PSM)

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Clinical Management of Dengue Haemorrhagic Fever (DHF)

• Hospitalization; monitor for signs of shock
• Oral rehydration solution (ORS), fruit juices, electrolyte solutions
• Antipyretics: Paracetamol (avoid aspirin and NSAIDs - risk of bleeding)
• Watch for warning signs: abdominal pain, black stools, bleeding gums, sudden BP drop
• Regular monitoring of haematocrit and platelet count
• IV fluid therapy when haemoconcentration rises significantly

• Immediate IV fluid resuscitation: Isotonic crystalloids (Ringer's lactate or normal saline) at 10-20 mL/kg over 15-30 min
• Reassess after each bolus; titrate fluids based on haematocrit, urine output, vital signs
• If haematocrit continues to rise despite fluids → colloids (gelatin, dextran) or blood transfusion
• Monitor: hourly urine output (target ≥1 mL/kg/hr), pulse pressure, BP
• Platelet transfusion: only for severe bleeding with very low platelets (<10,000 or active serious bleeding)
• Treat complications: metabolic acidosis, electrolyte imbalances, hypoglycaemia
• The critical period is the transition from febrile to afebrile phase (usually after day 3)

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Integrated Approach to Dengue Control

1. Vector control (most important):
   • Source reduction: Eliminate Aedes breeding sites (drain stagnant water, cover water containers, clean coolers weekly)
   • Biological control: Larvivorous fish (Gambusia, Poecilia), Bacillus thuringiensis israelensis (Bti) larvae
   • Chemical control: Larval control (temephos/abate in water containers); adult control by indoor residual spraying and fogging during outbreaks (using pyrethoids/malathion)
   • Environmental management: Proper solid waste disposal, town planning
2. Surveillance: Early case detection, reporting, and mapping of outbreaks
3. Community participation: Community education on removing containers with stagnant water; "Clean Friday" campaigns
4. Health education: Public awareness about symptoms, early treatment-seeking, avoiding self-medication with aspirin
5. Personal protection: Repellents, protective clothing, mosquito nets (especially during daylight)
6. Case management: Proper clinical protocols to reduce case fatality

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Question 3: Differentiate between Screening Test and Diagnostic Test

• Sensitivity = True Positives / (True Positives + False Negatives) × 100 → ability to correctly identify diseased persons
• Specificity = True Negatives / (True Negatives + False Positives) × 100 → ability to correctly identify non-diseased persons
• Positive Predictive Value = True Positives / (True Positives + False Positives) × 100
• Negative Predictive Value = True Negatives / (True Negatives + False Negatives) × 100

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Question 4: Measures for Prevention and Control of Air Pollution

A) Engineering/Technical Measures

1. Containment: Preventing escape of toxic substances into ambient air
   • Enclosure of industrial processes
   • Ventilation systems
   • Air cleaning devices and arresters (filters, scrubbers, cyclone separators, electrostatic precipitators) to remove particulate and gaseous pollutants
2. Replacement: Replacing polluting technologies with cleaner ones
   • Shifting from coal to electricity, solar power, natural gas, and CNG
   • Use of unleaded petrol (removing lead - a cumulative poison)
   • In India, deleaded petrol has been introduced
   • Electric vehicles replacing petrol/diesel engines
3. Dilution: Using the atmosphere's natural self-cleaning capacity
   • Establishment of green belts (vegetation corridors) between industrial and residential areas
   • Tall chimneys (better dispersion, NOT a solution - only dilution)
   • Has limits - atmosphere becomes overburdened if pollution exceeds self-cleaning capacity

B) Legislative/Administrative Measures

1. Legislation:
   • The Air (Prevention and Control of Pollution) Act, 1981 - India's primary legislation
   • Sets standards for ambient air quality (National Ambient Air Quality Standards - NAAQS)
   • Regulates chimney heights, smokestack emissions
   • Creation of smokeless zones
   • Powers to local authorities for investigation, research, education
   • International equivalent: Clean Air Acts (UK, USA)
   • Vehicle emission standards (BS-VI/Bharat Stage norms in India)
2. International Action:
   • WHO established an international network for monitoring air pollution with:
     • 2 international centres (London, Washington)
     • 3 regional centres (Moscow, Nagpur, Tokyo)
     • 20 laboratories worldwide
   • Issues warnings when pollution thresholds are exceeded

C) Public Health Measures

1. Urban planning:
   • Locating industries downwind from residential areas
   • Zoning laws separating industrial and residential zones
   • Tree plantation programs and maintaining parks
2. Health education:
   • Educating the public and industry on pollution effects
   • Promoting use of cleaner fuels (LPG over biomass for cooking)
   • Anti-smoking campaigns (tobacco smoke = major indoor air pollutant)
3. Air disinfection methods (for indoor air):
   • Mechanical ventilation: reduces vitiated air and bacterial density
   • Ultraviolet radiation: effective in operation theatres, infectious disease wards (lamps shaded in upper room, near air inlets)
   • Chemical mists: Triethylene glycol vapors as air bactericides (effective against droplet nuclei)
   • Dust control: Oil application to hospital ward floors reduces airborne bacteria
4. Monitoring:
   • Continuous ambient air quality monitoring
   • CPCB (Central Pollution Control Board) and SPCB stations in India
   • Air Quality Index (AQI) reporting for public awareness

Note on marks: The images show questions labeled S.E. (Short Essay) - typically 5 marks each. Only one explicit allocation is visible: "(3+4+3)" for a vector/NVBDCP question at the top. Questions 3-10 are S.E. type (~5 marks), and Q11-20 appear to be short notes (~3-5 marks). All answers are calibrated accordingly.

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PSM Answers - Short Essay Questions (5 marks each)

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Q3. Differentiate between Screening Test and Diagnostic Test (5 marks)

• Sensitivity = TP/(TP+FN) × 100 → ability to detect all diseased persons (true positive rate)
• Specificity = TN/(TN+FP) × 100 → ability to identify all healthy persons (true negative rate)
• Sensitivity and specificity are inversely related
• An ideal screening test = 100% sensitive + 100% specific (rarely achieved in practice)

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Q4. Measures for Prevention and Control of Air Pollution (5 marks)

1. Containment - Prevent escape of toxins into air using enclosure, ventilation, air cleaning devices (filters, scrubbers, electrostatic precipitators, "arresters")
2. Replacement - Replace polluting processes with cleaner ones: CNG, electricity, solar power, natural gas instead of coal; use of unleaded petrol (deleaded petrol in India)
3. Dilution - Green belts between industrial/residential areas; tall chimneys for dispersion (limited - atmosphere can be overburdened)

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Q5. List components of Diarrheal Disease Control Programme. Describe preventive strategies. (5 marks)

1. Case management (ORT cornerstone)
2. Health education and behaviour change communication
3. Water supply and sanitation improvement
4. Nutrition improvement
5. Training of health workers
6. Surveillance and monitoring

• Safe water supply - protected sources, household water purification
• Sanitation - proper disposal of human excreta, use of latrines
• Food safety - proper cooking, storage; avoid stale food; hand washing before food preparation
• Personal hygiene - hand washing with soap (especially after defecation and before eating)
• Breastfeeding promotion - exclusive breastfeeding for 6 months provides protection
• Complementary feeding - proper weaning practices to prevent malnutrition
• Fly control; proper waste disposal

• Oral Rehydration Therapy (ORT) - cornerstone of treatment; ORS (WHO formula: glucose 13.5 g, NaCl 2.6 g, KCl 1.5 g, trisodium citrate 2.9 g in 1 litre water)
• Zinc supplementation - 20 mg/day for 10-14 days in children (reduces severity and duration)
• Continued feeding during diarrhea (do NOT withhold food)
• Antibiotic therapy only for specific causes (cholera, dysentery)
• Referral for severe dehydration requiring IV fluids

• Management of complications (severe dehydration, electrolyte imbalances, malnutrition)
• Rehabilitation nutrition after illness

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Q6. Explain the primary level of prevention with suitable example (5 marks)

• Adequate nutrition and balanced diet
• Personal hygiene and environmental sanitation
• Exercise and healthy lifestyle
• Health education
• Proper housing, income support
• Marriage counselling, genetic counselling
• Periodic selective examination

• Immunization - e.g., vaccination against polio, measles, DPT, tetanus
• Use of specific nutrients - e.g., iodized salt to prevent goitre; Vitamin A to prevent night blindness
• Chemoprophylaxis - e.g., INH prophylaxis for TB contacts; chloroquine for malaria
• Protection from carcinogens - e.g., safety helmets, protective clothing
• Condom use - prevention of STIs including HIV
• Banning teratogenic drugs in pregnancy
• Fluoridation of water to prevent dental caries

Vaccination of children against measles is specific protection (primary prevention). It prevents the disease from occurring. Similarly, hand washing with soap before eating is health promotion, a non-specific measure that reduces multiple enteric diseases.

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Q7. Discuss the Physical Quality of Life Index (PQLI) (5 marks)

1. Infant Mortality Rate (IMR) - per 1000 live births
2. Life Expectancy at age 1 (not at birth) - in years
3. Literacy rate - percentage of literate adults

• Each component is scaled 0 to 100 (0 = worst performance, 100 = best performance)
• PQLI = Average of the three scaled values (equal weightage to each)
• Final PQLI is also on a scale of 0 to 100
• The ultimate objective is to attain PQLI = 100

• PQLI does NOT include per capita GNP, showing "money is not everything"
• Oil-rich Middle East countries have high income but not necessarily high PQLI
• Sri Lanka and Kerala (India) have low per capita income but high PQLIs - due to good social policies
• PQLI measures results of social, economic and political policies, not economic growth
• It complements but does not replace GNP

• Only 3 indicators - ignores nutrition, housing, employment, political freedom
• Does not reflect income distribution
• Life expectancy at age 1 is unusual (age at birth more standard)
• Replaced in modern usage by Human Development Index (HDI) (UNDP, 1990)

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Q8. Describe the Iceberg Phenomenon with suitable examples (5 marks)

      /\          ← TIP (visible) = Clinical cases seen in hospital/practice
     /  \
    /    \         ← SUBMERGED PART (hidden) = Sub-clinical cases,
   /______\          undiagnosed cases, carriers, latent infections

• Clinically manifest, diagnosed disease
• Patients attending hospitals and clinics
• What the physician sees in practice

• Sub-clinical / presymptomatic cases - disease process occurring but no symptoms yet
• Undiagnosed cases - symptomatic but not yet diagnosed
• Carriers - harbouring the agent without symptoms
• Latent infections - dormant disease (e.g., latent TB)

• Hidden cases act as reservoir of infection and source of spread
• Disease burden is severely underestimated from hospital data alone
• Justifies screening programmes to detect hidden disease
• Challenges preventive medicine to identify and control the submerged portion
• Epidemiologists study disease in the community - not just hospitals - to get a true picture

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Q9. Enlist different levels of health care. Compare and contrast services provided under these levels. (5 marks)

Level 1: Primary Health Care (PHC Level)

• Sub-centre: 3,000-5,000 (plain) / 1,000-3,000 (hilly/tribal)
• PHC: 20,000-30,000
• CHC: 80,000-1,20,000

• Promotive and preventive care (immunization, health education, family planning)
• Basic curative care (OPD, first aid, treatment of common illnesses)
• Maternal and child health (ANC, delivery, postnatal care)
• School health services
• Environmental sanitation, safe water supply
• Nutrition services (supplementary feeding, Vit. A, iron)
• Control of locally endemic diseases
• Treatment of minor ailments by ANM, MPW, ASHA

Level 2: Secondary Health Care

• Specialist care: medicine, surgery, obstetrics, paediatrics, orthopaedics, ENT
• Emergency services and casualty
• Inpatient care and surgical facilities
• Diagnostic services (lab, radiology, ECG)
• Referral from PHC level; back-referral to PHC

Level 3: Tertiary Health Care

• Super-specialist care (cardiology, neurosurgery, oncology, transplant surgery)
• Complex diagnostic procedures (MRI, CT, cath lab, PET scan)
• Research and training of health professionals
• Referral from secondary level
• Technology-intensive, expensive care

Comparison Table:

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Q10. Measures of Central Tendency (5 marks)

1. Mean (Arithmetic Mean)

• Definition: Sum of all observations divided by the number of observations
• Formula: x̄ = Σx / n
• Example: Data: 4, 6, 8, 10, 12 → Mean = 40/5 = 8
• Advantages: Uses all data values; amenable to algebraic manipulation; most stable
• Disadvantages: Affected by extreme values (outliers); not suitable for skewed data or open-ended distributions

2. Median

• Definition: The middle value when observations are arranged in ascending or descending order
• For odd n: Middle value; For even n: Average of two middle values
• Example: Data: 4, 6, 8, 10, 12 → Median = 8; Data: 4, 6, 8, 10 → Median = (6+8)/2 = 7
• Advantages: Not affected by extreme values; useful for skewed distributions and open-ended classes
• Disadvantages: Does not use all values in calculation; less stable than mean

3. Mode

• Definition: The value that occurs most frequently in a dataset
• Example: Data: 4, 6, 6, 8, 10 → Mode = 6
• Advantages: Unaffected by extremes; represents the most typical value; can be used for qualitative data
• Disadvantages: May not be unique (bimodal); unstable; doesn't use all values

Comparison:

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Q11. Describe Web of Causation with example (3-5 marks)

• Ideally suited for chronic diseases where no single specific agent is known
• Disease is the result of interaction of multiple factors in a complex network
• Considers all predisposing factors of any type and their complex interrelationships with each other
• Visualized as a "web" - multiple causes interact and produce disease through interconnected pathways

• Removal of even one link in the chain may be sufficient to control disease (does not require eliminating all causes)
• Individual factors are not of equal weight - their importance is expressed as relative risk
• Provides a model showing multiple possible intervention points

Genetic factors → High cholesterol
Sedentary lifestyle → Obesity → Hypertension → 
Smoking → Atherosclerosis → Coronary artery disease → MI
Stress → ↑ Catecholamines → Vasospasm
Diabetes → Endothelial damage

• Lung cancer: smoking + occupational carcinogens + air pollution + genetic susceptibility
• Hypertension: salt intake + obesity + stress + heredity + alcohol

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Q12. Discuss the different modes of intervention in disease prevention (5 marks)

• Non-specific measures improving overall health
• Examples: nutrition, exercise, health education, housing improvement, income support

• Against a specific agent
• Examples: immunization, chemoprophylaxis, use of specific nutrients (iodized salt), protective equipment

• Halt disease progression; prevent complications
• Tools: Screening programmes, periodic health examinations
• Examples: PAP smear for cervical cancer, blood glucose screening for diabetes

• Prevent or limit disability once disease established
• Examples: treatment of diabetic foot to prevent amputation; leprosy - prevent deformity by early MDT

• Restore function to maximum possible level after disability
• Includes: physiotherapy, occupational therapy, social rehabilitation, vocational retraining
• Examples: prosthesis after amputation; speech therapy after stroke

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Q13. Measures of Morbidity in Epidemiology (5 marks)

1. Incidence Rate

• Definition: Number of new cases of disease occurring in a defined population during a specified period
• Formula: Incidence = (New cases during period / Population at risk at beginning) × 1000 (or 10^n)
• Measures risk of developing disease
• Used for acute diseases

2. Prevalence Rate

• Measures burden of disease
• Used for chronic diseases (TB, diabetes)

3. Attack Rate

• Used in outbreak/epidemic investigations
• = (Number of cases in a group exposed to risk / Total exposed) × 100
• Secondary Attack Rate (SAR) = (New cases among contacts / Total susceptible contacts) × 100

4. Incidence Density (Person-Time Incidence)

• Used when follow-up periods differ among participants
• = New cases / Total person-time at risk (person-years)

Other measures:

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Q14. Enumerate the criteria for judging causality (5 marks)

1. Strength of association - A strong association (high relative risk) is more likely to be causal. E.g., smokers have 9-10x risk of lung cancer.
2. Consistency - Association is repeatedly observed by different investigators, in different populations, at different times and places. E.g., smoking-lung cancer association found in multiple countries.
3. Specificity - One cause produces one effect; the association is specific to a particular disease and population. (Less rigid criterion today - many causes can produce one disease)
4. Temporality - The cause must precede the effect. Exposure must come before disease. This is the only absolute criterion.
5. Biological gradient (Dose-response relationship) - As exposure increases, risk of disease increases. E.g., more cigarettes/day = higher lung cancer risk.
6. Plausibility - The association makes biological sense with current knowledge. E.g., carcinogens in tobacco causing DNA damage.
7. Coherence - The causal interpretation should not conflict with known facts of natural history and biology of the disease.
8. Experimental evidence - Removal of the suspected cause reduces or eliminates disease. E.g., stopping smoking reduces lung cancer risk.
9. Analogy - Similar associations exist for other disease-exposure pairs. E.g., thalidomide causing foetal abnormalities supports drug-teratogen associations.

Note: Temporality is the only essential criterion. The more criteria satisfied, the stronger the causal inference.

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Q15. Classify Disease Carrier with examples (5 marks)

Classification:

1. Temporary / Transient carrier - Carries the agent for short periods (days to weeks)
   • Example: Typhoid contacts during incubation period
2. Chronic carrier - Carries agent for months, years, or lifetime
   • Example: Chronic HBsAg carriers, chronic typhoid carriers (Mary Mallon - "Typhoid Mary"), chronic meningococcal carriers

1. Incubatory carrier - Carries and sheds agent during incubation period (before symptoms)
   • Example: Measles, mumps, chickenpox, influenza
2. Convalescent carrier - Continues to shed agent during recovery phase
   • Example: Typhoid, cholera, dysentery (during convalescence)
3. Healthy (passive) carrier - Never shows clinical disease despite harbouring agent
   • Example: Diphtheria (throat carriers), meningococcal infection, polio (99% inapparent)
4. Paradoxical carrier - A recovered case who is more dangerous as carrier than the original case

1. Urinary carrier - Shed via urine (typhoid, leptospirosis)
2. Intestinal carrier - Shed via faeces (typhoid, cholera)
3. Throat/Nasopharyngeal carrier - Shed via respiratory secretions (diphtheria, meningococcus)

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Q16. Define Adverse Events Following Immunization (AEFI) (3 marks)

1. Vaccine product-related reaction - Due to inherent properties of the vaccine (e.g., pain/swelling at injection site after DPT)
2. Vaccine quality defect-related reaction - Due to vaccine defect in manufacturing (e.g., improperly inactivated vaccine)
3. Immunization error-related reaction - Due to improper handling, preparation, or administration (e.g., wrong dose, wrong site, bacterial contamination from multi-dose vial)
4. Immunization anxiety-related reaction - Due to anxiety about immunization (e.g., vasovagal syncope)
5. Coincidental event - Occurs after immunization but not caused by vaccine (temporal association only)

• BCG adenitis after BCG vaccine
• Febrile seizure after DPT
• Intussusception after older rotavirus vaccine
• Anaphylaxis (rare) after any vaccine

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Q17. Types of Immunity (3-5 marks)

A. Innate (Natural/Non-specific) Immunity

• Present from birth; does not require prior exposure
• Non-specific; does not distinguish between pathogens
• Components: Skin/mucous membranes (barriers), phagocytes, NK cells, complement, interferons, fever

B. Acquired (Adaptive/Specific) Immunity

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Q18. Measures of Central Tendency (3 marks)

1. Mean = Sum of values ÷ Number of values (x̄ = Σx/n). Affected by outliers. Best for normally distributed data.
2. Median = Middle value when data is arranged in order. Not affected by extreme values. Best for skewed distributions.
3. Mode = Most frequently occurring value. Can be used for qualitative/nominal data.

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Q19. Enumerate methods of purification of water at household level (5 marks)

Physical Methods:

• Most reliable method; kills all pathogens
• Boil for at least 1 minute (3 minutes at high altitudes)
• Does NOT remove chemical pollutants; can cause re-contamination if stored improperly
• Economically accessible

• Fill transparent PET bottles, expose to sunlight for 6 hours (2 days if cloudy)
• UV-A and heat inactivate pathogens
• Low cost, no chemicals needed

• Slow sand filter - Schmutzdecke (biological layer) removes pathogens
• Ceramic pot filter - Locally made, effective for bacteria
• Candle filter (Berkefeld filter) - Porous ceramic; removes bacteria and protozoa
• Membrane filters (0.22 µm) - Remove bacteria; need pre-filtration

Chemical Methods:

• Chlorine tablets (e.g., Halazone, Aquatabs) - convenient for household use
• Sodium hypochlorite solution (bleach): 2 drops of 5% bleach per 1 litre water; contact time 30 min
• Effective against bacteria and viruses; less effective against Cryptosporidium

• Oxidizing agent; kills bacteria; used at 0.1% solution
• Less reliable than chlorination

• Tetraglycine hydroperiodide tablets
• Not for long-term use (thyroid effects); do not use in pregnancy

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Q20. Classify Insecticides (3-5 marks)

A. Based on Chemical Nature:

• Mechanism: Disrupt sodium channel in nerve membranes (persistent depolarization)
• Examples: DDT, BHC (HCH/lindane), Dieldrin, Aldrin, Chlordane
• Characteristics: Highly persistent in environment; bioaccumulate; now banned/restricted
• DDT still used for malaria control (indoor residual spraying) in some countries

• Mechanism: Inhibit acetylcholinesterase enzyme → accumulation of ACh → cholinergic toxicity
• Examples: Malathion, Parathion, Chlorpyrifos, DDVP (Dichlorvos), Fenthion, Temephos (Abate)
• Temephos used for larval control in drinking water containers (safe for humans)
• Less persistent in environment than organochlorines

• Mechanism: Also inhibit acetylcholinesterase (reversible inhibition)
• Examples: Carbaryl (Sevin), Propoxur (Baygon)
• Less toxic to mammals; less persistent

• Mechanism: Prolong sodium channel opening → nerve excitation
• Examples: Permethrin, Deltamethrin, Cypermethrin, Lambda-cyhalothrin
• Used in: Insecticide-treated bed nets (ITNs/LLINs), indoor residual spraying, sprays
• Low mammalian toxicity; biodegradable

• Mechanism: Act on nicotinic acetylcholine receptors
• Examples: Imidacloprid, Thiamethoxam
• High selectivity for insects; low mammalian toxicity

B. Based on Mode of Action:

1. Stomach poisons - Ingested (e.g., DDT, arsenicals)
2. Contact poisons - Absorbed through cuticle (e.g., pyrethroids, organophosphates)
3. Fumigants - Inhaled as vapour/gas (e.g., methyl bromide, DDVP)
4. Systemic insecticides - Absorbed by plant, insect ingests when feeding (e.g., imidacloprid)

C. Based on Target Stage:

1. Larvicides - Kill larvae: Temephos, Bti (Bacillus thuringiensis israelensis), oil larvicide
2. Adulticides - Kill adult insects: Malathion, pyrethroids (fogging, IRS)
3. Ovicides - Kill eggs (less common)

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All answers sourced from Park's Textbook of Preventive and Social Medicine and standard PSM curriculum. Answer depth is calibrated to 5 marks for S.E. questions and 3 marks for short note questions.