Carbon dioxide (CO2) is often in the news as the leading contributor to human-induced climate change. Since the start of the Industrial Revolution, the burning of fossil fuels, land clearing, and other human activities have led to an increase in outdoor CO2 from ~280 parts per million (ppm) to 417 ppm. Humans and animals also exhale CO2 as a normal part of their metabolism, and CO2 levels in occupied buildings can be several times higher than outdoors. CO2 levels in homes frequently exceed 1000 ppm and can sometimes approach 3000 ppm.
For decades, the conventional wisdom among indoor air specialists was that at these levels, CO2 itself is not toxic. But because humans constantly exhale it, it could act as a proxy for “bioeffluents”—the odors, droplets, and particles that humans give off as they breathe and perspire. By ventilating to keep CO2 low, other, harder-to-measure bioeffluents could be diluted to acceptable levels.
The arrival of a new and particularly nasty bioeffluent—the COVID-19 virus—has spurred interest in indoor CO2. Since CO2 and the virus are both exhaled in human breath, increasing ventilation to lower CO2 can also reduce rates of COVID-19 transmission.
It’s long been known that, at very high concentrations, CO2 is an asphyxiant. Levels exceeding 40,000 ppm are an immediate life hazard; OSHA sets the 8-hour permissible exposure at 5000 ppm. In the last 15 years, studies have suggested that the CO2 levels routinely encountered in homes may themselves impact human health and performance. CO2 concentrations above 1000 ppm, the threshold long recommended for managing bioeffluents, can decrease performance on cognitive tests. Moderately elevated CO2 has also been linked to reduced sleep quality, fatigue, elevated blood pressure, and sick-building syndrome.
The need for an HRV
My first experience managing indoor CO2 came several years ago at my company office. We built the 1000-sq.-ft.…
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