Air is supplied at \( 8 \ \mathrm{L/s} \) per person to 25 occupants. The total outdoor air flow is
Answer: C
The total is \( 8 \times 25 = 200 \) litres per second, and multiplying by 3.6 converts to \( 720 \ \mathrm{m^3/h} \). Mixing litres per second with cubic metres per hour is the commonest arithmetic slip in ventilation work.
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A room of \( 80 \ \mathrm{m^3} \) has an infiltration rate of 0.5 air changes per hour. A source of \( 40 \ \mathrm{mg/h} \) gives a steady concentration of
Answer: D
Correct answer: \( 1.0 \ \mathrm{mg/m^3} \)
Find the flow from the air exchange rate first:
\[ Q = nV = 0.5 \times 80 = 40\ \mathrm{m^3/h} \]
\[ C = \frac{G}{Q} = \frac{40}{40} = 1.0\ \mathrm{mg/m^3} \]
The two-step form is the usual one in practice, since infiltration is measured as an exchange rate while sources are quoted as mass per hour. Note that volume enters only through the flow; a larger room at the same exchange rate would ventilate proportionately more and reach the same concentration.
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Measured mixed air is 30 per cent outdoor air and 70 per cent recirculated. If outdoor carbon dioxide is 400 and return air is 1000 parts per million, the supply concentration is
Answer: A
Correct answer: 820 parts per million
The supply is a weighted average of the two streams:
\[ 0.3 \times 400 + 0.7 \times 1000 = 120 + 700 = 820\ \mathrm{ppm} \]
The calculation runs in reverse in practice: measuring the outdoor, return and supply concentrations gives the outdoor air fraction, which is how a commissioning engineer checks that a damper is delivering what it should. The distractor 700 uses only the recirculated stream, and 1400 adds instead of averaging.
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A house has \( 250 \ \mathrm{m^3} \) volume and needs 0.35 air changes per hour for adequate ventilation. The required flow is
Answer: B
Correct answer: \( 87.5 \ \mathrm{m^3/h} \)
Multiply the exchange rate by the volume:
\[ Q = 0.35 \times 250 = 87.5\ \mathrm{m^3/h} \]
The figure of 0.35 air changes per hour is a common minimum for dwellings, set to dilute the moisture and pollutants of ordinary occupancy. Older leaky houses often exceeded it by infiltration alone; well-sealed modern ones frequently do not, which is why mechanical ventilation became necessary as envelopes tightened.
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Statement I: Indoor concentrations of many pollutants exceed outdoor concentrations.
Statement II: Indoor sources release into a small volume with limited air exchange.
In the light of the above statements, choose the correct answer.
Answer: C
Correct answer: Both Statement I and Statement II are correct
Both are true and the second explains the first. An indoor source releases into a few tens of cubic metres exchanged perhaps once an hour, where an outdoor source releases into an effectively unlimited volume with constant mixing.
The same emission therefore produces a far higher concentration indoors. Since people spend most of their time indoors, indoor exposure often dominates total exposure - which is why indoor air quality is treated as a subject in its own right rather than as a corner of ambient work.
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Statement I: Carbon dioxide is measured indoors as an indicator of ventilation adequacy.
Statement II: Carbon dioxide is acutely toxic at the concentrations found in occupied rooms.
In the light of the above statements, choose the correct answer.
Answer: D
Correct answer: Statement I is correct but Statement II is incorrect
The first statement is true: carbon dioxide is measured as a surrogate for ventilation, because occupants generate it at a predictable rate and its excess over outdoor therefore reflects the outdoor air supplied per person.
The second is false. Indoor levels of 1000 to 2000 ppm are not acutely toxic; the significance of a high reading is that other occupant-related pollutants are being diluted just as poorly. It is an indicator, not a hazard at these concentrations.
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Statement I: Radon control focuses on preventing soil gas entry rather than on filtration.
Statement II: Radon is a noble gas that filters cannot capture.
In the light of the above statements, choose the correct answer.
Answer: A
Correct answer: Both Statement I and Statement II are correct
Both are true and the second is the reason for the first. Radon is a noble gas: it is chemically inert, so adsorbents have little hold on it, and it is a gas, so particulate filters pass it entirely.
Control therefore acts on entry rather than on the air already indoors - sealing cracks and service penetrations, and above all sub-slab depressurisation, which draws soil gas from beneath the floor and vents it above the roof so that it never enters at all.
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Statement I: House dust mite allergen can be reduced by controlling indoor humidity.
Statement II: Mites take up water vapour directly from the air rather than by drinking.
In the light of the above statements, choose the correct answer.
Answer: B
Correct answer: Both Statement I and Statement II are correct
Both are true and the second explains why humidity is the control. Mites absorb water vapour directly from the air through their cuticle, having no way to drink.
Below about 50 per cent relative humidity they cannot maintain their water balance and the population declines. That makes dehumidification and ventilation effective where cleaning alone is not, since allergen persists in dust long after the mites have gone - both are needed, but only humidity control prevents the population returning.
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Statement I: Sick building syndrome cannot be attributed to a single identified agent.
Statement II: Its symptoms are specific and diagnosable.
In the light of the above statements, choose the correct answer.
Answer: C
The first is correct and the second reverses the definition: the symptoms are deliberately described as non-specific. Building related illness is the term reserved for conditions with an identified agent and a diagnosis.
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Statement I: Local exhaust ventilation uses less air than dilution for the same protection.
Statement II: Capturing a contaminant at its source removes it before it mixes with the room air.
In the light of the above statements, choose the correct answer.
Answer: D
Correct answer: Both Statement I and Statement II are correct
Both are true and the second is the reason for the first. Local exhaust captures the contaminant while it is still concentrated at the source, so a modest flow through a well-placed hood removes the whole emission.
Dilution must reduce the concentration after the contaminant has mixed through the room, which needs a far larger volume of air for the same result - and that air must be heated or cooled, so the energy saving is substantial as well.
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