CO2: Indoor Air Consequences
October 1, 2007
In this white paper, take the scenic route to the topic with a quick, interesting overview of CO2 and its “life” in health and society thus far. Then move on to a discussion of managing CO2 in the building, the details of proactive and reactive technologies, and the VOCs, PMx and other contaminants that today’s facilities contend with.
CO2 is a colorless, odorless gas. It is produced when almost any form of carbon is burned in an excess of oxygen. It is released into the atmosphere during forest fires and the combustion of fossil fuels. Other natural sources of CO2 include volcanic eruptions, decay of dead plant and animal matter, evaporation from the oceans, and respiration by living plants and animals. It is removed from the atmosphere by CO2 “sinks,” (e.g., absorption by seawater and photosynthesis by ocean-dwelling plankton and land-dwelling biomass, including forests and grasslands). It is present in blood as a dissolved gas, bound to hemoglobin, or as carbonate/bicarbonate buffer of pH.
CO2 has no acute (short-term) health effects associated with low-level exposure (<5,000 ppm) typically found in indoor environments. CO2 behaves chiefly as a simple asphyxiate and narcotic, but it is not physiologically inert, being the most powerful cerebral vasodilator known. Inhaling large concentrations (hypercarbic, or elevated inspired CO2) causes rapid circulatory insufficiency (of oxygen) leading to coma and death. CO2 is also used for euthanasia and anesthesia of laboratory animals. Abnormal levels of CO2 in the body (hypercapnia) is caused by hypoventilation, lung disease, or diminished consciousness, by exposure to environments containing abnormally high levels of CO2, or by re-breathing exhaled CO2.
The main effect of CO2 involves its ability to displace oxygen within a confined space. As oxygen is inhaled, CO2 levels build up in the confined space, with a decrease in oxygen content in the available air. In certain industries, such as “dry” ice manufacture, brewing, baking, and soft drink manufacture, CO2 is incorporated into these products. Its safe intended uses are carefully monitored, as are recreational exposures, such as diving and cave exploring. Being 1.5 times as dense as air, CO2 accumulates in low areas. Incidental exposure to CO2 is an occupational concern for municipal workers in tunnels, sewers, and wastewater treatment plants, and for astronauts, submariners, and miners.
Environmental concerns have addressed both natural and anthropogenic sources of CO2. Until the emergence of halogenated compounds, CO2 was the “safe refrigerant,” since the principle alternatives, sulfur dioxide and ammonia, were noxious. CO2 also has been used extensively in fire extinguishing systems especially for electrical fires. Increased emissions of CO2, a greenhouse gas, have been indicted as contributing to global warming, i.e., increases in atmospheric and surface temperatures of the Earth, with potential environmental consequences, like melting of polar icecaps, altered plant growth, and climate change. In an extreme case, an upwelling of CO2 from a deep African Lake (Lake Nyos) caused the deaths of thousands of nearby inhabitants.
Historical justifications for setting CO2 levels in indoor workspaces have been subject to extended debate associated with various complaints such as stale air, stuffiness, body odor, discomfort, “Sick Building Syndrome” (SBS), etc. Groups such as ACGIH, ASHRAE, ANSI, and OSHA set standards on CO2 levels in buildings.
Increased levels of CO2 in indoor air are related fundamentally to building occupancy and building activities. Because CO2 is a natural product of respiration, the level in indoor air is proportional to the number of occupants as well as ventilation adequacy. CO2 has been incorporated as a surrogate diagnostic for other occupant generated pollutants, particularly biopollutants, and to evaluate air exchange rates and overall IAQ. It has been touted as a gross surrogate for the myriad of trace, less-definable components of indoor air simply because it is easy to understand and to measure, as compared to quantitating the collective health risks posed by numerous (and often unknown) trace components. Three questions are posed: Why do we ventilate? Why do we ventilate the way we do? and How much ventilation is needed? Ventilation is expressed in four paradigms (Fanger, 1998). The first paradigm, operating through most of the 19th century, assumed that people exhaled a highly toxic substance that required ventilation to avoid poisoning. Identification of this toxin ranged from CO2 to “anthropotoxin” (a hypothetical substance). The second paradigm, operating in the early 20th century, assumed that people emitted “contagion.” Disease-causing organisms were diluted by ventilation to decrease risk. The third paradigm gradually evolved during the 1920s to ’30s. Factors other than ventilation were found to be more important in the transfer of contagion. This shift toward “comfort” demanded dilution of human bioeffluents (respiration, halitosis, perspiration, flatulence), and tobacco smoke. The fourth paradigm directed toward the building as a system is now underway.
Implications for IAQCurrent efforts focus on avoiding or reducing superfluous pollution sources in buildings using engineered solutions directed specifically at controlling specific pollutants. Volatile organic compounds (VOCs) and particulate matter (PMX) from diverse sources [products of anyone’s fertile imagination (PAFIs) of the past] are now assuming real identities in the present. Conventional IAQ parameters of temperature, humidity, and airflow, are now expanded to include VOCs, PMX, and bioaerosols. IAQ is a balance between perception and reality. Ventilation (air handling) and remediation (air cleaning) must both be assessed. Buildings and occupants are continually assaulted by a potpourri of VOCs, particulates, and bioaerosols that affect IAQ. This diversity of air constituents challenges the health and productivity of building occupants, depreciates building values, and degrades manufactured goods.
Development of engineered solutions to mitigate IAQ problems requires an understanding of several factors: the types, sources, and interactions of building contaminants; the nature and activities of building occupants; the design and operation of building HVAC equipment; and the impact of indoor and outdoor climate.
Control strategies can be both passive and active. Passive control strategies include implementation of proactive measures to limit sources and exposures to potential contaminants. Active control strategies include reactive responses to actual contaminants through management of ventilation and air cleaning systems. An ordered hierarchy of control strategies includes source control, exposure control, ventilation, and air cleaning (EPA, 1990; ALA, 1997). The management of air, especially engineering design for indoor environments, requires interpretations of perception and reality (Daniels, 1999; Daniels, 2000).
What do we want to accomplish?
- Perception: a safe, healthy air environment free of dangerous, toxic, harmful, hazardous constituents
- Reality: a workable compromise balancing benefits, needs, and of course, inevitably, costs
Traditional monitoring methods and conventional air cleaning technologies do not always ensure consistent and flexible management systems. Building managers and health/environmental professionals must choose among prevention and remediation technologies to ensure clean air for all purposes.
Proactive (preventive) technologies restrict accumulation of VOCs, deposition of particles, and proliferation of microbes on surfaces. Reactive (mitigating) technologies destroy volatiles, agglomerate particulates, and inactivate airborne microbes. Air cleaning can include both "before" and "after" approaches.
A proactive technology applies specific silane-coupling chemicals to form non-migrating antimicrobial coatings onto building surfaces. These coatings pemanently sorb and bond to surfaces, thereby preventing colonization and proliferation of biofilms.
A reactive technology uses a titanium dioxide catalyst and UV light to generate hydroxyl radicals that react with airborne contaminants through processes of UV photocatalytic oxidation (UVPCO) and UV germicidal irradiation (UVGI).
Automatic monitoring and reporting extends the usefulness of proactive/reactive technologies to more types of buildings and indoor air constituents, ranging from normal activities to catastrophic events. Rapid assessments of operation and performance of engineered systems for air cleaning require real-time monitoring and logging of significant IAQ parameters (CO2, CO, temperature, rh, TVOC, PM2.5, PM10, O3, Rn). Evaluation and reporting is done using representative data collected in real-time and automatically uploaded via wireless technology to a secure, Web-based IAQ knowledge center.
The real issue in managing building airflows is not just controlling CO2, but focusing on the control of the "trace" components and applying revolutionary new technologies, like photocatalytic oxidation. New, energy-efficient houses are now so tight that most leaks have been eliminated. As CO2 levels rise in response to low outside air supplies, so will levels of trace contaminants. Supplying more outside air dilutes the level of CO2 and the levels of trace components, but does not remove trace contaminants. In the past, consultants in the IAQ industry would simply say, "Dilution is the solution to indoor air pollution!" In the future, consultants and practitioners can say, "Revolution is the solution to indoor air pollution!" ES
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