Competing needs for energy efficiency and occupant health, safety and comfort come into play in the design and operations of HVAC systems. Current practices tend to be weighted to energy savings with arguably less recognition of the potential trade-off costs to human health and productivity.
The most recent 2010 edition of ASHRAE standard 62.1, Ventilation for Acceptable Air Quality, provides an illustration. It specifies 17 cubic feet per minute (cfm) of fresh air per occupant as the minimum acceptable ventilation level for office space – a drop from the standard’s previous minimum of 20 cfm per occupant.
In sync, sophisticated building automation systems are now effectively controlling ventilation rates at these lower levels, so it’s hardly surprising that HVAC system designers, consultants and operations personnel have begun to target that number as an optimally efficient ventilation rate. However, indoor air quality specialists tend not to agree.
In establishing 17 cfm as the minimum acceptable level for offices, the ASHRAE standard explicitly uses a short-term subjective rationale to determine acceptability: “Acceptable indoor air quality (is) air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80 per cent or more) of the people exposed do not express dissatisfaction.”
In application, this means air quality is deemed acceptable if four out of five visitors to the space are not immediately offended by odours or other contaminants. Phrased somewhat differently, it means the minimum acceptable ventilation standard is one in which every fifth visitor to an office space is offended by unpleasant smells.
The standard is also clear that subjective assessment cannot detect other potentially threatening contaminants that are neither odoriferous nor noticeably irritating, which might include radon, mould, bacteria and/or non-irritating volatile organic compounds (VOCs) that can compromise human health and productivity at elevated levels. This is not a fault in the ASHRAE standard per se but, rather, a risk inherent in interpreting the 17 cfm minimum guideline as an acceptable standard in itself.
Fresh air remedy
Simply increasing the fresh air ventilation rate is one of the best and easiest ways to reduce indoor air quality contaminants. Airborne contaminants are primarily generated within the building and can be dispersed by exchanging inside air with fresh outside air.
Beyond health, there is an economic rationale. A Harvard University study has estimated cost savings related to employee productivity in the order of $400 US per employee per year (in 2000 dollars) simply by doubling the fresh air ventilation rate. Other research suggests the value in gained productivity far exceeds the potential flipside energy cost savings.
The optimal ventilation level will vary based on factors such as building and mechanical design, occupant densities, human resources considerations and seasonal energy costs. Flu and pandemic season also calls for special consideration since increasing the ventilation airflow will help minimize the spread of microbiological contamination.
Even energy-efficient buildings with finely controlled building automation systems will likely have some excess capacity in the ventilation systems as a matter of best practice. Meanwhile, increased ventilation might be justified as a best practice from health, productivity and financial perspectives. Health and wellness professionals should be consulted in addition to energy management specialists.
Curt LaMontagne is an indoor environmentalist and principal of C5 Plus Ltd., a firm specializing in indoor environmental science and technology.