how air moves in a building
Just like people, every building needs to breathe. If you are to have a healthy and productive
indoor environment, a constant flow of new air has to come in and replace the old air, which
needs to be released outdoors.
How much a building needs to breathe depends on a number of things. What type of building
it is. Where it is located. What kind of activities it is used for. How many people use each room
at any given time. And so on.
How air enters a building
The controlled way to bring air inside is through the ventilation systems’ supply air vents.
However, air also enters via so-called passive ventilation or infiltration. This occurs when
we open doors and windows, and through minor leaks around doors and windows when
they are closed and via gaps around pipes, vents and chimneys. The air exchange is
additionally affected by the outside temperature and how much the wind blows.
The effects of the Stack Effect
The Stack Effect is how air moves (stacks) inside. It occurs due to warm air being lighter
than cold air. When the outside temperature is cold, the warm indoor air rises and creates
a difference in pressure that makes colder air come in (infiltrate) via doors, windows and
other leakage points at the bottom of the building. When the temperature outside is warm,
the Stack Effect is reversed and the warmer air enters via the upper levels creating a
Rates vary between countries
Recommended ventilation rates often vary quite significantly between countries. A study
of a number of European countries showed that one group of countries had ventilation rates
of around 10 l/s and another group of countries had 4 l/s.
Another study (in the United States) of how indoor air quality impacts people indicated that
when the ventilation rate was increased from 20 to 40 cfm (10-20 l/s) of outdoor air per person,
it led to an 8 percent increase in employee decision making. Or about $6,500 in increased
productivity per person per year.
Ventilation ensures the air is safe
Ensuring a satisfactory ventilation rate, is not just about people’s general well-being and
productivity. Without proper ventilation, the indoor air can become dangerous. It can lead
to a buildup of deadly levels of carbon monoxide and harmful concentrations of radon.
Radon is a naturally occurring, radioactive, odourless and invisible gas that is harmless
outdoors but has the potential to accumulate to hazardous levels indoors. Exposure to
radon can cause cancer.
New buildings are more energy efficent
Newer buildings are usually tighter and therefore need a ventilation system with greater
capacity for air exchange compared to older houses. The upside is that newer buildings
are more energy efficient. Older constructions have more leakage through passive ventilation
(infiltration), which leads to higher energy costs.
When a building functions, for example, as an office, a school, a library or a hospital, it is
wise to invest in air purifiers to help the ventilation system take care of air pollution from
In industrial indoor environments, the ventilation system is often supplemented by hoods
and extractors that capture air pollution at the source. It can be welding dust, oil mist, wood
dust, etc. Industrial operations also benefit from purifying the indoor air using products such as
Camfil’s industrial air cleaners and Camfil’s dust, fume and mist collectors.
- Ventilation Rate - the amount and quality of the outdoor air that is allowed inside
- Airflow Direction - the direction of the overall airflow from zones with clean air to zones
with dirty air
- Air Distribution/Airflow Patterns - the efficiency with which the outdoor air is distributed
to each indoor area from where indoor airborne pollutants should be efficiently removed
Ventilation rate is the rate at which the air is exchanged, it can be specified as:
- Liters/Second/Person (l/s)and per Square Meter of floor area
- Cubic Feet/minute (cfm) per person and per square feet of floor area
Reducing health issues and costs
A well functioning ventilation system with high grade air filters can:
- reduce the number of children who develop asthma or allergies by half. WHO estimates
that 20% of the world’s population suffers from allergic diseases, many of whom are children.
- lower your heating costs by 30 to 40 percent.
WHO roadmap to healthy ventilation
The global COVID-19 pandemic has put the spotlight on the importance of indoor air quality
(IAQ), especially when it comes to reducing the spread of viruses indoors. To provide support
with IAQ, WHO has published a roadmap outlining the necessary steps to ensure healthy
ventilation levels in healthcare and non residential settings.