---
title: "Ventilation standards and fresh-air requirement"
subject: "Environment and Pollution Control"
chapter: 3
key: "semester_7/environment_and_pollution_control:ventilation-standards-co2-numerical"
source: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/ventilation-standards-co2-numerical
site: BCT Engineering
---

# Ventilation standards and fresh-air requirement

Environment and Pollution Control · Chapter 3: Indoor Air Quality

- Asked in: year not given, 3+5 marks
- Video: [How to Calculate Ventilation Air](https://www.youtube.com/watch?v=mnURGW-WFTs) — MEP Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/environment_and_pollution_control/ventilation-standards-co2-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_7/environment_and_pollution_control/chapter-3

## Question

(a) What is the purpose of ventilation standards? State two ways in which they are specified. (b) A classroom 6 m x 5 m x 3 m is occupied by 10 persons, each releasing 0.0052 L/s of CO2. The outdoor CO2 level is 400 ppm and the indoor level is to be kept at 1000 ppm. Calculate the fresh-air flow required (in m3/h and L/s per person) and the air changes per hour.

## Short answer

Using Q = G/(Ci - Co), generation is 0.052 L/s and concentration difference is 600 ppm, giving 0.0867 m³/s or 312 m³/h, which is 8.7 L/s per person. Room volume is 90 m³, so air changes per hour are 3.5.

## Answer

#### (a) Ventilation standards
A ventilation standard (e.g. ASHRAE 62.1) specifies the minimum outdoor air that must be supplied to a space so that the concentration of contaminants stays at a level acceptable to occupants. It protects health and comfort and is the basis for designing HVAC systems.

Ways of specifying:
1. **Ventilation rate procedure:** a prescribed airflow per person (L/s per person) plus a component per unit floor area (L/s per $\text{m}^2$) for building-related sources, depending on the use of the space.
2. **Air change rate:** the number of room volumes of fresh air supplied per hour (ACH), common for hospitals, kitchens and laboratories.
3. (Also) **Indoor air quality procedure:** limiting the concentration of selected contaminants (e.g. a CO$_2$ level about 700 ppm above outdoors, or total VOCs).

#### (b) Numerical
Mass balance at steady state: pollutant generation = removal by ventilation.
$$
G = Q\,(C_i - C_o) \Rightarrow Q = \frac{G}{C_i - C_o}
$$
Generation: $G = 10 \times 0.0052 = 0.052$ L/s $= 5.2\times10^{-5}\ \text{m}^3$/s.

Concentration difference: $C_i - C_o = (1000 - 400)\times10^{-6} = 6\times10^{-4}$.
$$
Q = \frac{5.2\times10^{-5}}{6\times10^{-4}} = 0.0867\ \text{m}^3/\text{s} = 86.7\ \text{L/s} = 312\ \text{m}^3/\text{h}
$$
Per person: $86.7/10 = 8.67$ L/s per person.

Room volume $= 6\times5\times3 = 90\ \text{m}^3$:
$$
\text{ACH} = \frac{312}{90} = 3.47\ \text{h}^{-1}
$$
**Answer: Fresh air required = 312 m$^3$/h (86.7 L/s, i.e. 8.7 L/s per person); ACH = 3.5 per hour.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The purpose of a ventilation standard is to:
   A. Specify the minimum outdoor air so contaminants stay at acceptable levels
   B. Maximise energy use
   C. Define building materials
   D. Set outdoor pollution limits
   Answer: A. It sets minimum outdoor air supply to protect health and comfort and fixes HVAC design. (Key point: Purpose of standards)

2. The steady-state ventilation relation is:
   A. Q = G x (Ci - Co)
   B. Q = G/(Ci - Co)
   C. Q = (Ci - Co)/G
   D. Q = G + (Ci - Co)
   Answer: B. Generation equals removal, G = Q(Ci - Co), so Q = G/(Ci - Co). (Key point: Mass balance)

3. G = 0.052 L/s and Ci - Co = 600 ppm give a fresh-air flow of:
   A. 8.67 L/s
   B. 86.7 L/s
   C. 867 L/s
   D. 3.12 L/s
   Answer: B. 0.052/6e-4 = 86.7 L/s = 312 m3/h, i.e. 8.67 L/s per person. (Key point: Fresh air flow)

4. With 312 m3/h and a 90 m3 room, ACH is about:
   A. 0.29
   B. 3.5
   C. 34.7
   D. 312
   Answer: B. 312/90 = 3.47 per hour. (Key point: Air changes)

## Related in Environment and Pollution Control

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- [Control strategies for indoor air quality](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aindoor-air-control-strategies) — Environment and Pollution Control, chapter 3
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