Wednesday, June 26, 2013

So you think you are safe from the haze indoors? Think again.

During the current haze many people assume they are safe indoors. Many who do not have air-conditioning at home have preferred to stay in their offices thinking that they are safer there.

To some extent, it may be true, in that they are less exposed to the full brunt of the haze and noxious chemicals. But they may be less protected than they think.

The design of building ventilation systems tend to concentrate more on the replacement of stale air than the exclusion on environmental pollutants. For example, the US EPA current requirements for ventilation systems in offices and schools are air exchange rates of at least 15-20 cubic feet per minute (Cfm) per person. This ensures that the building is not "sick" and will have adequate freshness of air. There is on the other hand much less in terms of expectation to provide a barrier to environmental pollutants.

In an enclosed building, air influx from external sources occurs primarily through ventilators. One assumes that some filtration occurs, but this is not mandated. Even if filters are present these cannot be those with too small pores as they will reduce the mechanical efficiency of the ventilators. Usually I believe, contractors just suck in air from the outside. This explains why many people complain of feeling the effect of the haze and smelling the smoke even in their offices.

How much contamination of office air occurs? This is hard to predict and measure. The smaller the particle, they less likely they will be excluded from the internal air. PM2.5 therefore tends to get everywhere, even though the contamination is less visible.

This level of contamination by outside air is indexed as the infiltration efficiency. A recent study in the US reported that across many areas, the infiltration efficiency of PM2.5 may be as much as 80%.  If this is true, there is not much benefit in hiding in your offices. Ironically, it may be that sicker buildings with poorer air exchange may actually be safer.

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