Between Scylla and Charybdis: energy, carbon dioxide, and indoor environmental quality


In Book XII of Homer's Odyssey, Ulysses navigates his ship through a narrow strait guarded on one side by Scylla, a six-headed monster, and on the other by Charybdis, a violent whirlpool. Rather than risk the loss of his entire ship, Ulysses passes too close to Scylla, leading to the deaths of six of his best men. Of key importance, Ulysses knew of the challenges ahead of him, having been warned in detail by Circe.

On two prior occasions, editorials in Indoor Air have explored the challenges and opportunities for indoor environmental quality that arise in connection with anthropogenic climate change (Nazaroff, 2008; Spengler, 2012). Three developments over the past year lead me to share with you some further thoughts.

First, in late 2012, the International Energy Agency released their annual update of the World Energy Outlook (IEA, 2012). This report made headlines, stating that, ‘by around 2020, the United States is projected to become the largest global oil producer’. The IEA also predicted that North America would become a net oil exporter by around 2030. The report included chilling news that did not make headlines here in the US: ‘the climate goal of limiting warming to 2°C is becoming more difficult and more costly with each year that passes’.

Second, in May 2013 came the announcement that atmospheric CO2 levels measured at the Mauna Loa Observatory in Hawaii exceeded – for the first time – the psychologically important threshold of 400 ppm. (As I write this editorial in mid May, the measured concentration has been revised to 399.89 ppm. So, it seems, the crossing of this limit is delayed for a short time.)

Third, a recently published experimental study explored the effects of short-term exposure to carbon dioxide on decision-making performance (Satish et al., 2012). This study found that ‘relative to 600 ppm, at 1000 ppm CO2, moderate and statistically significant decrements occurred in six of nine scales of decision-making performance'.

How are these three items related?

First, let us reiterate the connection between fossil energy use and atmospheric CO2 levels. When fossil fuels – coal, petroleum, or natural gas – are burned, carbon atoms that were long sequestered from the environment are released to the atmosphere as carbon dioxide. The chemical transformation is an inevitable consequence of extracting energy from the fuels. Discharging CO2 to the atmosphere, although not inevitable, is standard practice. Alternatives for carbon capture and sequestration (CCS) are being developed and demonstrated. However, one cannot anticipate widespread use of CCS in the near future.

A substantial fraction – roughly half – of all the carbon released from human use of fossil fuels has accumulated in the atmosphere, increasing the atmospheric abundance of CO2 from a preindustrial level of 280 ppm to the 2012 average of 394 ppm. The annual average CO2 level measured at Mauna Loa has increased every year since monitoring began there in 1958. The rate of increase has also trended upwards, from about 0.9 ppm/year during the 1960s to about 2.0 ppm/year for the first decade of the current century.

Carbon dioxide is the fifth most abundant species in the atmosphere (following N2, O2, Ar, and H2O). It is noteworthy that anthropogenic activities have substantially altered the abundance of such a prominent environmental species. Also worth noting, the current level of approximately 400 ppm is entirely unprecedented in the history of humankind on earth. Ice cores from Antarctica show that for the past 600,000 years the atmospheric CO2 level varied between 200 ppm and 280 ppm with lower levels corresponding to ice ages and higher levels to interglacial warming periods. The current rate of increase of 2 ppm/year of atmospheric CO2 corresponds to the net addition, day by day, of 1.7 kg of fossil carbon to the atmosphere for each of earth's seven billion people.

Concern for accumulating atmospheric carbon dioxide has focused first on the increases in mean environmental temperatures that are expected to result. The fourth assessment of the Intergovernmental Panel on Climate Change (IPCC, 2007) reported that a net radiative forcing of 1.66 W/m2 was associated with the change in atmospheric CO2 from the preindustrial level of 280 ppm to the 379 ppm average level as of 2005. Applied continuously over the whole earth's surface area (500 trillion m2), the net effective increase in energy absorption in the biosphere is 26 × 1021 J/y or 26 ZJ/y (where Z = zetta = 1021). This energy flow greatly exceeds the purposeful use of energy by humankind, roughly 0.5 ZJ/y. The internationally agreed upon target to limit warming to 2°C relative to the preindustrial baseline implies a cap on atmospheric CO2 levels somewhere in the range 450–500 ppm. For ‘business as usual’ scenarios as assessed by IPCC, the level of atmospheric CO2 in year 2100 is expected to be in the range 550–970 ppm.

Although the primary focus of increasing atmospheric CO2 has been on its contribution to climate change via radiative forcing, other environmental concerns have surfaced. Among these are the acidification of oceans with associated damage to marine ecosystems (Doney et al., 2009) and the potential for increased allergenic pollen releases (Wayne et al., 2002).

Aiming to mitigate anthropogenic climate change, all major aspects of society's energy supply and end use are being scrutinized. Considerable attention focuses on energy use in buildings, which are both a large energy-use sector and also responsible for very large fossil carbon emissions to the atmosphere. Since indoor environmental quality (IEQ) is not consistently good throughout the current building stock, it is possible that efforts to improve overall building performance might lead to win-win opportunities that also improve IEQ. However, a strong focus on reducing energy use without proper attention to the services provided for building occupants poses risks of degrading IEQ. A major study of the US Institute of Medicine (IOM, 2011) has explored this nexus between climate change, indoor environmental quality, and health.

In densely occupied spaces, IEQ is altered by human bioeffluents. A major aspect of the ventilation requirements for occupied spaces is tied to occupancy, typically expressed in terms of the volume of air to be provided per time per occupant. Carbon dioxide, a major product of human metabolism, is a primary bioeffluent. For a sedentary adult, the mean CO2 emission rate is about 34 g/h per person (ASHRAE, 2010). In poorly ventilated and densely occupied spaces, human metabolism leads to carbon dioxide levels exceeding 1000 ppm.

A long history documents that human bioeffluents erode air quality in poorly ventilated spaces. In a popular lecture published in 1873, the German scholar Max von Pettenkofer wrote (referring to carbon dioxide as ‘carbonic acid’), ‘A series of examinations have resulted in the conviction that one volume of carbonic acid in 1000 volumes of room air indicates the limits which divide good from bad air. This is now generally adopted and practically proved, always provided, that man is the only source of carbonic acid in the space in question' (Von Pettenkofer, 1873).

In reviewing the scientific literature through the late 1990s, Seppänen et al. (1999) wrote that, ‘21 studies, with over 30,000 subjects, investigated the association of carbon dioxide concentration with (human) responses. … About half of the carbon dioxide studies suggest that the risk of sick building syndrome symptoms continued to decrease significantly with decreasing carbon dioxide concentrations below 800 ppm’. Since then, several additional studies have been published that show associations between elevated metabolic carbon dioxide and sick building syndrome symptoms in offices (Erdmann and Apte, 2004), student absenteeism in schools (Shendell et al., 2004), and test performance in classrooms (Wargocki and Wyon, 2007; Haverinen-Shaughnessy et al., 2011; Twardella et al., 2012; Bakó-Biró et al., 2012).

As the literature has accumulated associating elevated indoor metabolic CO2 levels with some adverse outcomes, we should be asking the question: what are the causes? It is often assumed that CO2 is simply a marker for other (not yet isolated or identified) bioeffluents. Carbon dioxide is abundant and is relatively easy to measure; so it is a convenient marker compound. As far back as 1873, von Pettenkofer wrote, ‘I will not say that I consider the detected carbonic acid as the principal drawback to such (degraded) air; it is, in my mind, the measure only for all the other alterations which take place in the air simultaneously and proportionately, in consequence of respiration and perspiration; its increase shows to what degree the existing air has already been in the lungs of the persons present’.

But what if CO2 is not just a marker of other bioeffluents? What if exposure to elevated CO2 levels has adverse consequences?

Their contribution toward answering this question is what makes the recent work of Satish et al. (2012) so important. The study showed that even short-term exposure to moderately elevated CO2 levels of 1000 ppm, isolated from enhanced exposure to bioeffluents, led to measurable decrements in decision-making performance. Their subjects were also exposed to CO2 at 2500 ppm, for which some performance metrics decreased ‘to levels associated with marginal or dysfunctional performance’.

If society continues to rely heavily on fossil fuels to meet its energy needs, and if it continues to discharge the resulting CO2 to the atmosphere, then the ambient CO2 levels will continue to rise. Under these conditions, it will become progressively harder to maintain indoor CO2 levels below an absolute target value of, say, 1000 ppm. The challenge will be amplified with increasing urbanization, because fossil carbon emissions tend to be more heavily concentrated in cities, causing an elevation in the ambient CO2 levels in air over cities as compared to the background troposphere (George et al., 2007; Jacobson, 2010). If, on the other hand, society seeks to address anthropogenic climate-change concerns by severely reducing energy use in buildings, and if those energy reduction measures include lower ventilation rates, then we may directly experience exposures to higher indoor CO2 levels. Based on the findings of Satish et al., these outcomes might represent a choice between Scylla and Charybdis. A knowledge economy in particular and human wellbeing in general are not compatible with eroded decision-making performance.

However, our prospects are not inevitably bleak. The gap between ambient CO2 levels (400 ppm) and the levels found by Satish et al. (1000 ppm) to degrade decision-making performance is large in comparison with the annual increase in atmospheric CO2 level. So, there is time to change the path we are on. However, there is also a huge momentum in ‘business as usual’, and so it will take considerable effort to change direction and that change will not happen rapidly.

Research can contribute. More studies are needed to assess whether, how, and to what extent exposure to CO2 itself has adverse consequences for human wellbeing at levels normally encountered indoors. Improvements for more reliably and inexpensively monitoring CO2 levels would be welcome, especially if expanded to permit effective assessments of human exposures. Undoubtedly, opportunities exist to realize cobenefits with improved building energy performance and better ventilation effectiveness through stronger design, operation, and maintenance of components and systems. If CO2 exposure is indeed contributing to detrimental outcomes, then considerable near-term societal benefits might be realized by improving ventilation conditions in densely occupied spaces such as classrooms. In sum, there is a basis for hope that we can continue our journey without having to encounter Scylla or Charybdis.