Liam Brady

NDY Project Engineer

Feeling Sleepy? This may be why.

The effects of Carbon Dioxide are not commonly known throughout the industry despite the fact that we contribute much effort to maintaining low levels of this greenhouse gas. But how much does this small molecule affect your everyday work?

The State of Play

The impact of outdoor air in engineering design is a common narrative we engineers encounter and contend with most days. Generally speaking, higher quantities result in healthier occupants but consequently leads to higher energy usage and larger systems. Naturally, we try to reduce the available amount as much as legislation and building targets allow. Legislation for providing outdoor air has been slowly moving towards a goal of what appears to be activity driven, whatever that may be. But how much can we reduce the outside air supply before we see an impact on cognitive function of people in these spaces?

Legislators and peak body organisations have aimed to improve occupant health by providing credits for improved energy efficiency, environmental performance, and occupant wellbeing. For example, a calculation within ASHRAE 62.1determines the expected CO2,eqconcentration that should be maintained to satisfy the substantial majority (about 80%) of occupied spaces with respect to building ventilation.  Approximately, an outside air flow rate of 10L/s per person results in a concentration of around 520ppm above the ambient for sedentary working. For reference, atmospheric levels of CO2,eqare around 350ppm, although this may be higher if one is standing next to a cow paddock.

One part in a million

The Green Building Council of Australiaawards buildings that can ensure indoor CO2,eqlevels are maintained at acceptable levels 1 point for maintaining occupied spaces below 800ppm, and 2 points for below 700ppm. Sydney Universitystandards require occupied spaces to be kept below 800ppm and the new WELLstandard requires control to 800ppm for occupant densities over 25 people per 93m² (1,000ft²). Compare these benchmarks to a standard office, which will typically sit between 800 – 1200ppm, though it could reach up to 2000ppm in a tightly packed meeting room. In education buildings including primary schools, classrooms can exceed 2700ppm when unconditioned1.  Research has shown that at higher exposure to CO2people undergo significant reductions in decision making performance and concentration2, as well as health symptoms in younger occupants such as headaches or common colds3.

Finding the middle ground

An experiment published in Environmental Health Perspectives4illuminated a link between CO2,eqconcentrations and decreased cognitive functioning. A total of 24 participants spent 6 full working days in environmentally controlled spaces performing boring monotonous cognitive tasks whilst recording the scores while altering the local CO2,eqlevels per day. There were 3 categories of days where the CO2,eqconcentrations were varied to ‘Conventional’ (950ppm), ‘Green’ (750ppm), and ‘Green+’ (500ppm). Interestingly, this experiment aligns very closely to the targets being set by peak body organisations such as WELL and Greenstar.

A person’s basic activity level, described as the overall ability to make decisions at all times, will increase by approximately 50% when subject to CO2levels lower than 750ppm according to this study. The ability to plan, stay prepared, and strategise under emergency conditions, otherwise called Crisis Response, saw a 100% increase in ability with CO2levels kept at 750ppm when compared to Conventional levels whereas maintaining a CO2level at 500ppm showed a 235% increase in responsiveness.

By far the most increased task-based ability was synonymous with most office jobs; Information Usage. This was described as the capacity to use both provided information and information that has been gathered towards attaining overall goals. Participants showed an increase in scores by 170% on 750ppm days and a whopping 300% increase in ability for CO2levels at 500ppm.

There is always a juggle between succinct design and providing high quality work environments for occupants. Considering we spend around 90% of our time indoors it is our imperative as designers to help optimise our designs for both occupants and building owners. Decreasing the available outside air will decrease duct sizes, equipment costs, and operating costs however it can be seen that providing additional outside air can significantly impact the cognitive abilities of the occupants. Benchmarks set by peak body organisations of maintaining 800ppm or below demonstrates alignment with research to improve occupant’s wellbeing. If CO2concentrations can be maintained to these levels, employees and employers may see impressive benefits.

  1. Clements-Croome, M.Eftekhari, R. Greene, and G.Georgiou. 2012. Measurements of CO2Levels in a Classroom and its Effect on the Performance of the Students”. CIBSE ASHRAE Technical Symposium, Imperial College, London UK.
  2. William J. Fisk, Toshifumi Hotchi, Mark J. Mendell, Usha Satish, Krishnamurthy Shekhar, Siegfried Streufert, and Douglas Sullivan. 2012. “Is CO2an Indoor Pollutant? Direct Effects of Low-to-Moderate CO­2Concentrations on Human Decision-Making Performance”. Environmental Health Perspectives, Vol 120, December 2012: 1671–1677.
  3. Nkwocha E.E., Egejury R.O. “Effects of industrial air pollution on the respiratory health of children”. International Journal of Environmental Science and Technology. Vol 5, September 2008: 509–516.
  4. Joseph G. Allen, Piers MacNaughton, Suresh Santanam, Usha Satish, John D. Spengler, and Jose Vallarino. 2016. “Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments”. Environmental Health Perspectives, Vol 124, June 2016: 805–812.


This article originally appeared in the CIBSE ANZ journal:

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