Environmental Health — MCQs

Environmental Health Indian Medical PG Practice Questions and MCQs

Question 481: What percentage of bleaching powder is used to disinfect feces?

A. 2%
B. 5% (Correct Answer)
C. 10%
D. 15%

Explanation: **Explanation:** The correct answer is **5%**. Bleaching powder (Calcium hypochlorite) is a powerful oxidizing agent widely used for disinfection in public health. **1. Why 5% is correct:** Disinfecting excreta (feces and urine) requires a higher concentration of disinfectant due to the high organic load. Organic matter neutralizes chlorine rapidly. A **5% solution of bleaching powder** (with a contact time of at least 1 hour) is the standard recommendation for the disinfection of stools and vomit, especially during outbreaks of enteric diseases like Cholera. This concentration ensures that enough "free residual chlorine" remains active to kill pathogens despite the presence of organic debris. **2. Analysis of Incorrect Options:** * **2% (Option A):** This concentration is generally insufficient for solid waste or heavy organic loads. It may be used for less contaminated surfaces but is not the standard for feces. * **10% & 15% (Options C & D):** These concentrations are unnecessarily high for routine fecal disinfection. While effective, they are highly corrosive to containers, irritating to the respiratory tract, and economically wasteful. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chlorine Demand:** The difference between the amount of chlorine added and the amount of residual chlorine remaining after a specific contact time. * **Drinking Water:** For disinfecting drinking water, the standard dose is roughly **0.5 mg/L** (residual chlorine) after 30 minutes of contact time. * **Bleaching Powder Composition:** Fresh bleaching powder contains approximately **33% available chlorine**. It is unstable and loses chlorine content on exposure to air, light, or moisture. * **Horrocks’ Apparatus:** Used to estimate the dose of bleaching powder required to disinfect a given quantity of water. * **OT Test (Orthotolidine):** Used to determine both free and combined residual chlorine in water.

Question 482: All of the following are true regarding DDT, EXCEPT:

A. It acts as a neurotoxin.
B. It is primarily a contact poison.
C. It does not cause immediate death, but it takes several hours to kill.
D. It has repellent action on insects. (Correct Answer)

Explanation: **Explanation:** DDT (Dichloro-diphenyl-trichloroethane) is a classic organochlorine compound used extensively in public health programs for Indoor Residual Spraying (IRS). **1. Why Option D is the Correct Answer (The "Except"):** DDT **does not have a repellent action**. In fact, its lack of repellency is a strategic advantage; insects (like the *Anopheles* mosquito) do not avoid the sprayed surfaces. Instead, they rest on the treated walls, allowing the chemical to be absorbed through their legs. This is in contrast to certain pyrethroids which may exhibit repellent properties. **2. Analysis of Incorrect Options:** * **Option A (Neurotoxin):** DDT is a potent neurotoxin for insects. It works by opening sodium channels in the nerve cells, leading to repetitive firing of impulses, followed by paralysis and death. * **Option B (Contact Poison):** It is primarily a contact poison. It is absorbed through the cuticle (exoskeleton) of the insect when it comes into physical contact with a sprayed surface. * **Option C (Time to Kill):** DDT is not an "instant-kill" agent. It is a slow-acting insecticide. After contact, the insect undergoes a period of excitation and tremors (the "DDT jitters"), and death typically occurs several hours later. **High-Yield Clinical Pearls for NEET-PG:** * **Dosage:** The standard dose for IRS is **200 mg/sq. ft.** * **Residual Efficacy:** It remains effective on surfaces for **6 months**. * **Environmental Impact:** It is highly lipid-soluble and stable, leading to **biomagnification** in the food chain. * **Storage:** In humans, DDT is stored in **adipose tissue**. * **Resistance:** Widespread resistance in *Anopheles* mosquitoes has led to the transition to Malathion or Synthetic Pyrethroids in many regions.

Question 483: A sanitary well should be so located that no user will have to carry water for more than:

A. 100 yards (Correct Answer)
B. 500 yards
C. 1000 yards
D. 350 yards

Explanation: ### Explanation In Community Medicine, the criteria for a **Sanitary Well** are defined to ensure both the safety of the water source and its accessibility to the community. **1. Why 100 yards is correct:** According to standard public health guidelines (Park’s Textbook of Preventive and Social Medicine), a sanitary well must be located within a reasonable distance to ensure compliance with hygiene practices. The rule of thumb is that the well should be located so that no user has to carry water for more than **100 yards (approximately 91 meters)**. This distance is considered the maximum threshold to encourage the use of safe water; if the distance is greater, users are more likely to resort to closer, potentially contaminated sources (like ponds or streams). **2. Analysis of Incorrect Options:** * **B, C, and D (500, 1000, and 350 yards):** These distances are too far. Increased distance leads to "water fatigue," where the quantity of water brought into the home decreases, negatively impacting personal hygiene and increasing the risk of water-washed diseases. **3. High-Yield Clinical Pearls for NEET-PG:** * **Distance from Contamination:** A sanitary well must be located at least **15 meters (50 feet)** away from potential sources of pollution (latrines, soakage pits, or refuse heaps) to prevent seepage of pathogens. * **Lining:** The well should be lined with bricks or stones set in cement up to a depth of at least **6 meters (20 feet)** to prevent subsoil water from entering. * **Parapet Wall:** A parapet wall of at least **28 inches (70 cm)** height should be constructed to prevent surface washings and accidents. * **Platform:** A cement concrete platform should extend at least **3 feet** all around the well, sloping outwards towards a drain.

Question 484: In a slow sand filter, what is the element responsible for yielding bacteria-free water?

A. Valve
B. Vital layer (Correct Answer)
C. Supernatant water
D. Under-drainage system

Explanation: ### Explanation The **Slow Sand Filter** (also known as a biological filter) is a cornerstone of large-scale water purification. The correct answer is the **Vital Layer**, as it is the biological heart of the filtration process. **1. Why the Vital Layer is Correct:** Also known as the **Schmutzdecke**, zoogleal layer, or biological layer, this is a slimy, gelatinous film that forms on the surface of the sand bed within a few days of operation. It consists of algae, bacteria, protozoa, and plankton. This layer is responsible for the high efficiency of the filter because it removes organic matter, oxidizes ammoniacal nitrogen, and **removes 98–99% of bacteria** through biological action and adsorption. **2. Why Other Options are Incorrect:** * **Valve:** These are mechanical components used to regulate the flow of water into and out of the filter; they have no role in the actual removal of pathogens. * **Supernatant Water:** This is the column of raw water (usually 1–1.5 meters) above the sand bed. Its primary purpose is to provide a constant head of pressure to push water through the filter and provide waiting time for sedimentation; it does not filter bacteria. * **Under-drainage System:** Located at the bottom of the filter (porous pipes), its role is to provide an outlet for the purified water and support the filter medium. It does not contribute to the purification process itself. **High-Yield Facts for NEET-PG:** * **Rate of Filtration:** 0.1 to 0.4 $m^3/h/m^2$ (much slower than Rapid Sand Filters). * **Cleaning:** Done by **scraping** the top layer of sand (1–2 cm). * **Ripening of Filter:** The period (usually 2–3 days) it takes for the Schmutzdecke to form before the water is safe for consumption. * **Effective Size of Sand:** 0.2 to 0.35 mm. * **Comparison:** Unlike Rapid Sand Filters, Slow Sand Filters **do not** require chemical coagulation (alum).

Question 485: Which of the following is/are used as food preservatives?

A. Ethylene dibromide and Potassium meta-bisulphate
B. Sodium benzoate
C. Potassium meta-bisulphate and Sodium benzoate (Correct Answer)
D. Ethylene dibromide, Potassium meta-bisulphate, and Sodium benzoate

Explanation: **Explanation:** Food preservation is a critical public health measure used to inhibit microbial growth and prevent chemical changes that lead to food spoilage. In the context of Community Medicine and Food Hygiene, preservatives are categorized into Class I (natural) and Class II (chemical). **Why Option C is Correct:** * **Sodium Benzoate:** This is a widely used chemical preservative (Class II). It works most effectively in acidic conditions (pH < 4.5) by inhibiting the growth of yeasts, molds, and some bacteria. It is commonly used in squashes, syrups, and carbonated drinks. * **Potassium Meta-bisulphate (KMS):** This acts by releasing sulfur dioxide ($SO_2$) when added to food. It serves as both an antimicrobial agent and an antioxidant, preventing the browning of fruits and juices. **Analysis of Incorrect Options:** * **Ethylene Dibromide (EDB):** This is a **fumigant** and pesticide used to protect stored grains from insects. It is not a food preservative and is considered a potential carcinogen; its use is strictly regulated or banned in many food processes. Therefore, options A and D are incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Class I Preservatives (Natural):** Salt, sugar, vinegar, vegetable oils, and honey. * **Class II Preservatives (Chemical):** Sodium benzoate, Potassium meta-bisulphate, Sorbic acid, and Nitrates/Nitrites (used in cured meats). * **Sulfur Dioxide Caution:** KMS should not be used in naturally colored juices (like pomegranate or jamun) because $SO_2$ has bleaching properties that can destroy the natural color. * **Safety:** Sodium benzoate is generally recognized as safe (GRAS) but is limited to a maximum concentration of 0.1% in most food products.

Question 486: Residual chlorine is detected by which test?

A. Horrock's test
B. Ortho-toluidine test (Correct Answer)
C. Chromatography
D. Polarimeter

Explanation: **Explanation:** **1. Ortho-toluidine (OT) Test (Correct Answer):** The Ortho-toluidine test is the standard method used to determine both **Free and Combined Residual Chlorine** in water. When OT reagent is added to water containing chlorine, it produces a yellow color. The intensity of the yellow color is proportional to the concentration of chlorine, which is then measured by comparing it against standard color discs in a **chloroscope**. * *Note:* To specifically differentiate between free and combined chlorine, the **Ortho-toluidine Arsenite (OTA) test** is used. **2. Why other options are incorrect:** * **Horrock’s Test:** This is used to estimate the **chlorine demand** of water (i.e., how much bleaching powder is required to disinfect a specific volume of water), rather than measuring the residual amount left after disinfection. * **Chromatography:** This is a laboratory technique used for separating mixtures (e.g., identifying drug metabolites or toxins) and is not used for routine water chlorination monitoring. * **Polarimeter:** This instrument measures the rotation of polarized light by optically active substances (like sugars); it has no application in water chemistry. **High-Yield Clinical Pearls for NEET-PG:** * **Contact Time:** For effective chlorination, the recommended contact time between chlorine and water is **60 minutes**. * **Residual Chlorine Level:** The standard recommended level of free residual chlorine in drinking water is **0.5 mg/L** after 1 hour of contact. * **Post-Disaster:** In the event of a cholera outbreak or disaster, the level should be increased to **1.0 mg/L**. * **Flash Test:** A modification of the OT test where the reading is taken immediately (within 10 seconds) to measure only Free Residual Chlorine.

Question 487: What is the most satisfactory method of refuse disposal?

A. Dumping
B. Controlled tipping (Correct Answer)
C. Incineration
D. Manure pits

Explanation: **Explanation:** **Controlled Tipping (Sanitary Landfill)** is considered the most satisfactory and hygienic method of refuse disposal. It involves depositing waste in a low-lying area, compacting it, and covering it daily with a layer of earth (at least 6 inches). This method is preferred because it is environmentally safe, prevents fly and rodent breeding, and eliminates odors. Over time, the organic matter undergoes chemical and bacteriological decomposition, eventually converting the waste into stable humus. **Why other options are incorrect:** * **Dumping:** This is the most primitive and unsatisfactory method. Open dumping attracts flies, rodents, and scavengers, causes air pollution through spontaneous combustion, and leads to surface and groundwater contamination. * **Incineration:** While effective for reducing volume and destroying pathogens (ideal for hospital/biomedical waste), it is expensive and causes significant air pollution. It is not the "most satisfactory" for general municipal refuse due to high operational costs and loss of organic matter. * **Manure Pits:** These are primarily used in rural areas for animal dung and agricultural waste. They are not suitable for large-scale urban municipal refuse disposal. **High-Yield Pearls for NEET-PG:** * **Methods of Controlled Tipping:** Trench method (flat terrain), Ramp method (sloping terrain), and Area method (depressions/quarries). * **Temperature:** During decomposition in a landfill, temperatures rise to **60°C (140°F)** within 7 days, killing most pathogens. * **Completion:** It takes approximately **4–6 months** for complete decomposition into stable mass. * **Biomedical Waste:** Remember, while controlled tipping is best for municipal waste, **Incineration** is the gold standard for anatomical waste (Yellow bag).

Question 488: Which of the following is the best parameter to measure air pollution?

A. SO2 (Correct Answer)
B. CO2
C. CO
D. N20

Explanation: **Explanation** **Why SO₂ is the correct answer:** Sulfur Dioxide (SO₂) is considered the best indicator of air pollution, particularly for **urban air quality**, because it is a byproduct of burning fossil fuels (coal and oil) containing sulfur. It serves as a "proxy" for general industrial pollution. High levels of SO₂ are directly linked to severe health outcomes, such as exacerbations of asthma, bronchitis, and the formation of "acid rain." In the context of public health monitoring, SO₂ levels are used globally to assess the effectiveness of clean air policies. **Analysis of Incorrect Options:** * **CO₂ (Carbon Dioxide):** While a major greenhouse gas responsible for global warming, it is not considered a primary "pollutant" for measuring immediate air quality or toxicity, as it is a natural component of the atmosphere and human respiration. It is, however, an indicator of **indoor air freshness/ventilation**. * **CO (Carbon Monoxide):** This is a highly toxic gas resulting from incomplete combustion. While it is a significant pollutant, it is usually localized to heavy traffic areas and is not used as the *broad* parameter for general air pollution levels compared to SO₂. * **N₂O (Nitrous Oxide):** This is primarily a greenhouse gas and an anesthetic agent. While nitrogen oxides (NOx) are pollutants, N₂O specifically is not the standard parameter for measuring general atmospheric pollution. **High-Yield Pearls for NEET-PG:** * **Best Indicator of Indoor Air Pollution:** CO₂ (levels should not exceed 0.1% or 1000 ppm). * **Indicator of Smoke/Particulate Matter:** Soot or "Black Smoke." * **Lichens:** These are sensitive biological indicators; they disappear in areas with high SO₂ levels. * **Air Quality Index (AQI):** In India, the National AQI monitors 8 pollutants: PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃, and Pb.

Question 489: Purification of water at a large scale is achieved by all the following methods except:

A. Disinfection of wells (Correct Answer)
B. Rapid sand filtration
C. Slow sand filtration
D. Storage

Explanation: **Explanation:** The purification of water on a **large scale** (town or city level) typically follows a systematic three-stage process: **Storage, Filtration, and Disinfection.** **Why "Disinfection of Wells" is the correct answer:** Disinfection of wells is a method used for **small-scale (local) water purification**, specifically for rural or individual water supplies. It is not a method used to supply water to large urban populations. While it involves chemical treatment (usually with bleaching powder), it lacks the preliminary stages of sedimentation and filtration required for large-scale municipal distribution. **Analysis of Incorrect Options:** * **Storage (B):** This is the first step in large-scale purification. Storing water for 10–14 days allows for physical settlement of suspended impurities (sedimentation) and reduces the bacterial count by up to 90% through natural oxidation and UV exposure. * **Slow Sand Filtration (Biological Filter):** Used for large-scale purification, it utilizes a "Schmutzdecke" (vital layer) to remove 98-99% of bacteria. It is highly effective but requires a large land area. * **Rapid Sand Filtration (Mechanical Filter):** The standard method for modern cities. It involves coagulation, flocculation, and rapid filtration. It is faster than slow sand filtration and requires less space. **High-Yield NEET-PG Pearls:** * **Chlorination:** The most common method for large-scale disinfection. The "Contact Time" required is at least 30-60 minutes. * **Horrocks’ Apparatus:** Used to estimate the dose of bleaching powder needed to disinfect a well. * **Orthotolidine (OT) Test:** Used to detect free and combined chlorine in water. * **Vital Layer:** Also known as the *Zoogleal layer*, it is the heart of the Slow Sand Filter.

Question 490: All of the following are types of mechanical ventilation except?

A. Perflation and aspiration (Correct Answer)
B. Exhaust ventilation
C. Plenum ventilation
D. Air conditioning

Explanation: **Explanation:** Ventilation is categorized into two main types: **Natural** and **Mechanical (Artificial)**. **1. Why "Perflation and Aspiration" is the correct answer:** Perflation and aspiration are mechanisms of **Natural Ventilation**. * **Perflation:** This occurs when wind blows through open doors and windows, creating a through-current of air. * **Aspiration:** This occurs when air moving at a high velocity over an opening (like a chimney or window) creates a low-pressure zone, "sucking" the inside air out. Since these rely on natural wind forces rather than mechanical fans, they are not types of mechanical ventilation. **2. Analysis of Incorrect Options (Types of Mechanical Ventilation):** * **Exhaust Ventilation (B):** Uses fans to extract vitiated air from a room, creating a vacuum that draws fresh air in through inlets. * **Plenum Ventilation (C):** Also known as "Propulsion ventilation," where fans push fresh air into the room, creating positive pressure that forces stale air out. * **Air Conditioning (D):** The most advanced form of mechanical ventilation, which provides simultaneous control of temperature, humidity, and air purity. **High-Yield Clinical Pearls for NEET-PG:** * **Standard of Ventilation:** The most common parameter used to measure the efficiency of ventilation is the **Air Change Rate** (number of times the air is replaced per hour). * **Overcrowding:** In India, a floor area of **less than 50-100 sq. ft. per person** is often used as a criterion for overcrowding. * **Kata Thermometer:** Used to measure the "cooling power" of air and low wind velocities. * **Anemometer:** Used specifically to measure high wind velocity.

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