AIR AND VENTILATION 1 POLLUTION AIR AND VENTILATION


Many kinds of ventilation system are challenged by poor-quality external air. Tim Dwyer looks at the lessons provided by a CIBSE guide, below, while on page 52 we look at how air movement via louvres can be made more effective


The challenge of keeping internal environments acceptable and safe can apply to all sorts of ventilation


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hether it is for a naturally ventilated design, mechanical ventilation or mixed mode, the challenge


is still the same – to provide an internal environment that is both acceptable and safe for the occupants. The external air quality largely determines the opportunity to introduce ‘fresh air’ into a building system. Despite improvements over recent decades, air pollution is still seen as a challenge in the UK. So the implications of poor air quality are


clear, and having access to the best quality of outdoor air must be a consideration in the design of ventilation systems. CIBSE TM21: Minimising pollution at air intakes divides air pollution into ‘gaseous’ and ‘particulate’, and these are further sub-divided into two categories, primary and secondary pollutants. The table on


the next page provides an abstracted set of pollutants commonly associated with indoor air quality. The actual prediction of any pollutant


can be a challenge but – unlike primary pollutants that are emitted directly by the polluting process, such as vehicle emissions or a local toilet ventilation discharge – secondary pollutants by their very nature are more diffi cult to forecast. They are generated by breakdown reactions involving the primary pollutants and are dependent upon a coincident set of conditions. For example, levels of ‘ground-level’


ozone will depend on chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. In urban areas, ozone concentrations will be impacted by a combination of exhaust from


March 2012 CIBSE Journal 47


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