Carbon Dioxide
Carbon dioxide is produced as part of the metabolic
process and has an emission rate that is dependent on the level of activity.
While CO2 is not normally harmful, the concentration of CO2
can act as a marker for the adequacy of ventilation. Following the
commencement of occupation in a room, the concentration rises to an
'equilibrium' or 'steady state' concentration (see dilution
ventilation).
Provided there are no other sources of CO2, the
per occupant rate of ventilation may be estimated from this steady state
concentration. Assuming sedentary occupation (e.g. office workers), the Figure
(right) relates ventilation rate in litres per second per person (l/s.p)
with the steady state CO2 concentration. Several authorities
specify a maximum indoor CO2 concentration for occupied spaces
of between 800 - 1000ppm. From the Figure this equates to a minimum
ventilation rate of approximately 7.5 - 10 l/s.p. This approach forms the
basis of CO2
demand controlled ventilation systems.
CO2 concentration rises most rapidly in densely occupied
areas such as schools and theatres (i.e. where the source strength to volume
ratio is high). In large or sparsely occupied buildings, steady state levels
may never be reached. As a rule, if the measured CO2concentration
is found to be above the desired target value (e.g. above 1000ppm), then the
ventilation rate is not at the required amount. On the other
hand, if the CO2 concentration is below the target value, the adequacy of ventilation is not
necessarily confirmed, since it is possible that the steady state value has
yet to be reached. |
Caveats
 Strictly, it is the difference between the
indoor and outdoor CO2 concentration that provides a measure of
metabolic impact. Typically, target concentrations are based on 350 - 400 ppm
outside concentration. If the local outdoor value is substantially outside this
range, then a correction should be made to compensate for the difference (i.e.
add or subtract the difference from the target concentration).
The above only applies when metabolism is the
sole source of CO2 production. Gas cookers and flueless combustion
appliances, for example, emit large amounts of CO2 into a space.
For areas in which intensive metabolic
activity takes place (e.g. gymnasiums), the metabolic CO2 production rate
increases substantially. CO2 threshold levels may therefore be
expected to rise considerably above the thresholds set for sedentary
activities.
The fact that CO2 concentration may
rise above typically accepted threshold levels (e.g. 800 - 1000ppm) does not
necessarily imply a health risk. Concentrations of 5000 - 10000 ppm may often
regarded as acceptable although comfort conditions will usually deteriorate
considerably.
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