This significant difference in the findings was attributed to the fact that smoke particles tend to coagulate together into larger particulates as they age. According to Lougheed, "This suggests that the aging of smoke occurs more rapidly than might be expected; in ductwork it occurs predominantly within the first few metres" (p. 3). The analysis of the HVAC air filters indicated that there were some specific considerations that should be taken into account when selecting air filter types and in their placement to avoid adversely affecting the efficacy of the duct-mounted smoke detectors. In this regard, Lougheed concludes that the placement of air filters in return air ducts is not a concern; however, the author emphasizes that, On the supply side, the duct smoke detector is placed after the fresh air inlet, the filter, the conditioning area, and the fan. In this case, the filters would have an impact on the detection of fires (other than a fire in the filter itself), as they can reduce the smoke to a level at which it is not a concern" (p. 4). Notwithstanding this latter constraint, Lougheed suggests that based on the overall study findings, duct-mounted smoke detectors will provide warnings in those cases where the smoke concentrations in the HVAC system reach those levels established for safe building evacuation. In addition, other experiments...
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