Haze is coming again, Is my building central airconditioning ventilation and air filtration systems sufficient?

This involves building aircon, fan and ventilation design. Hope this will help the Facilities, Aircon Technicians, Engineers and Managers.


Haze particles are unburnt particles, soot. The haze smoke is made of very small particles gaseous molecules to very big particles. Big particles of about >10um, Fine particles of 1-10um, Very fine particles (0.3um), Odour gases/vapours.

PM2.5, it refers to concentration microgram per litre for particles less than micron size below 2.5um. 

The souce of polluted contaminated air comes from outside, as buildings are enclosed limit fresh air intakes, doors or windows openings or gaps.

PM2.5 and dust spot efficiency particle size

They are related but not exactly linked because building M&E aircon and ventilation design is crucial for good indoor air quality. PM2.5 refers to concentration of particles <2.5um. Air filtration refers to percentage of filtration of no of particles. The indoor air quality shall be affected by aircon and ventilation design, source of contaminants including  food, furniture, rubbish etc. The recent ISO16890 to replace EN779 relates the indoor air qualtiy with filtration to ePM2.5, ePM10.0 but different suppliers may present it differently. The comparison to one another is similar and does not make an impact. It will easier to use ASHRAE MERV for faster selections.MERV6 (G3) prefilters and MERV14 (F8) secondary filters for the control of haze particle with recommended optional carbon filters to use which also recommended in SS553.

Buildings aircon or AHUs work like a giant purifiying, exchanging air. Good ventilation design, maintenance will eventually get good air change which is crucial which means faster recirculation and cleaning the air. During Haze, the number of particles in the air is tremendous. Normal healthy air is 35-80ug/m3. During Haze, it peaks around >200ug/m3. In layman terms to relate, Most Building AHUs with MERV13 80-85%@1-3um will expect more than 95% removal of contaminants (PM2.5) removed. With MERV14, it clears about > 98% of PM2.5 particles.

MERV14 Filtration or Dust spot Eff 90-95% F8 >90% at 1-um, >95%@3-10um, Arrestance >5um, 99% by weight.

Example. Outside worst case 400ug/l - 95% removal = Estimated 20ug/m3 in buildings. Good healthy <100ug/m3.  Air filtration efficiencies increase as it loads with dust! of course expect lesser air to pass through.

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