Which one of the following statements correctly defines PM1 particles in air?
- (a)These are suspended particles of diameter more than 10 micron
- (b)These are the particles that are filtered by the nose during respiration
- (c)These are extremely fine particles of diameter less than 1 micron
- (d)These are coarse particles that penetrate directly through the lungs into the bloodstream
Correct — C, these are extremely fine particles of diameter less than 1 micron. In air-quality work the letters PM stand for particulate matter and the number that follows is the upper limit of aerodynamic diameter in micrometres. PM10 means particles up to 10 micrometres across, PM2.5 up to 2.5 micrometres, and PM1 up to 1 micrometre. The classes are nested: every PM1 particle is also counted within PM2.5 and PM10. Because PM1 particles are the finest of the three, they travel deepest into the lungs, reach the alveoli and can cross into the bloodstream, and they stay airborne longest.
- (a)These are suspended particles of diameter more than 10 micron — Particles larger than 10 micrometres are outside the PM10 class altogether and are usually called coarse or total suspended particulates. The PM number is a ceiling, not a floor.
- (b)These are the particles that are filtered by the nose during respiration — Nasal hairs and mucus filter out the coarser fraction, typically above about 10 micrometres. PM1 is a hundred times finer than that and passes the nose untouched, which is precisely why it is a health concern.
- (d)These are coarse particles that penetrate directly through the lungs into the bloodstream — This option is self-contradictory. Coarse particles are stopped in the upper airways; it is the fine and ultrafine fraction, not the coarse one, that can reach the alveoli and enter the blood.
Airborne particulate matter is classified by aerodynamic diameter because that is what decides how deep a particle penetrates the respiratory tract. Coarse particles above 10 micrometres are trapped in the nose and throat, the 2.5 to 10 micrometre fraction reaches the bronchi, and particles below 2.5 micrometres reach the alveoli, where gas exchange takes place. Below 1 micrometre the particles behave almost like a gas, remain suspended for days and can enter the bloodstream.
Three of the four options are built on the same misconception, that a smaller PM number means a bigger particle or a less dangerous one. Fix the convention once and the item is instant: the number is the ceiling in micrometres, and smaller numbers mean finer, more dangerous particles. For scale, a human hair is about 70 micrometres across, so a PM1 particle is roughly one seventieth of its width. India's National Ambient Air Quality Standards, notified in 2009, prescribe limits for PM10 and PM2.5; there is no notified national standard for PM1, which is monitored in research rather than in routine compliance.
- PM stands for particulate matter, and the number is the upper aerodynamic diameter in micrometres.
- PM1 is under 1 micrometre, PM2.5 under 2.5 micrometres, PM10 under 10 micrometres; the classes are nested.
- Particles below about 2.5 micrometres reach the alveoli; the finest of them can cross into the bloodstream.
- A human hair is roughly 70 micrometres across, about seventy times a PM1 particle.
- India's National Ambient Air Quality Standards of 2009 set limits for PM10 and PM2.5, not for PM1.
The number is a ceiling, so a smaller number means a finer and more hazardous particle.
- Reading the PM number as a lower limit rather than an upper one.
- Assuming the nose filters fine particles as effectively as it filters dust.
- Confusing aerodynamic diameter with physical diameter for irregular particles.
A definition item. The examiner offers one correct definition and three inversions of it, so knowing the convention behind the number is the whole of the work.
In the context of WHO Air Quality Guidelines, consider the following statements: 1. The 24-hour mean of PM2.5 should not exceed 15 μg/m³ and annual mean of PM2.5 should not exceed 5 μg/m³. 2. In a year, the highest levels of ozone pollution occur during the periods of inclement weather. 3. PM10 can penetrate the lung barrier and enter the bloodstream. 4. Excessive ozone in the air can trigger asthma. Which of the statements given above are correct?
- (a) 1, 3 and 4
- (b) 1 and 4 only
- (c) 2, 3 and 4
- (d) 1 and 2 only
Answer(b) 1 and 4 only
The same size-versus-depth rule, tested as a false statement. Its third statement fails precisely because PM10 is too coarse to cross the lung barrier, while the finer fractions can — which is the property that defines PM1 here.
- practice — not a real PYQ
PM2.5 in air-quality reporting refers to particles of aerodynamic diameter
- (a)exactly 2.5 micrometres
- (b)less than 2.5 micrometres
- (c)more than 2.5 micrometres
- (d)between 2.5 and 10 micrometres
Answer(b) less than 2.5 micrometres — the number is the upper limit of diameter.
- practice — not a real PYQ
Which one of the following particle sizes is most likely to reach the alveoli of the lungs?
- (a)50 micrometres
- (b)20 micrometres
- (c)12 micrometres
- (d)1 micrometre
Answer(d) 1 micrometre — the coarser fractions are trapped higher in the airway.