Correct Option
The correct option is (c).
Explanation
Air is a mixture of various gases, water vapour, and solid particles known as particulate matter or dust. While the composition of gases remains relatively stable in the lower atmosphere, the concentration of dust particles is highly variable depending on environmental and anthropogenic factors.
Statement-wise Analysis
- Statement 1 is Correct: Dust particles are an integral component of the atmosphere. They originate from various sources such as soil erosion, volcanic eruptions, pollen, industrial emissions, and vehicular exhaust. Consequently, some amount of particulate matter is always suspended in the air.
- Statement 2 is Incorrect: The concentration of dust particles varies significantly across different times and places. For instance, air in industrial cities or construction zones has a much higher dust content compared to rural areas or gardens. Similarly, dust levels can fluctuate based on weather conditions (e.g., rain washes down dust) and time of day (e.g., peak traffic hours).
- Statement 3 is Correct: The human nasal cavity is lined with fine hairs and mucus that act as a natural filtration system, trapping dust and other foreign particles to prevent them from entering the respiratory system. Breathing through the mouth bypasses this filtration mechanism, allowing harmful dust particles to enter the lungs directly, which can lead to respiratory issues.
Key Takeaway: While dust is a permanent constituent of air, its concentration is spatially and temporally variable. Nasal breathing provides a physiological defense against inhaling particulate matter, unlike mouth breathing.