Poor indoor-air quality (IAQ) and elevated carbon dioxide (CO2) levels correlate to discomfort and productivity loss in a wide range of environments, according to scientific and medical researchers.
Trace pollutant CO2 impairs cognitive and respiratory functions. Sustained concentrations exceeding 600-700 parts per million (ppm) over outside levels are associated with inadequate ventilation. Monitoring CO2 concentration serves as a convenient proxy for overall indoor-air health.
The U.S. Environmental Protection Agency (EPA) has identified poor IAQ as one of the five most urgent environmental risks to public health, yet facility managers and building engineers responsible for ensuring optimal ventilation in work areas remain unfamiliar with the best developed methods of controlling CO2 levels. Wireless data loggers measure CO2 together with temperature and humidity, providing real-time awareness that is essential to enforcing high environmental standards.
With long-term monitoring, managers gain insights that support better decisions regarding ventilation control and HVAC upgrades, and these projects can lead to energy savings and improved IAQ. Comprehensive, location-specific CO2 data in building environments focuses HVAC improvements on effective, cost-efficient solutions.
Battery-powered CO2 data loggers measure indoor concentrations. The compact hand-held devices are roughly the size and shape of a wall-mounted home thermostat and mount wherever CO2 data is needed. Measurement typically ranges from 0 to 5,000 ppm. Options allow access to data from mobile devices and quick data downloads directly to a laptop or from the cloud.
Clearing the air
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) defines acceptable indoor-air quality as “air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80 percent or more) of the people exposed do not express dissatisfaction.”
A key marker of IAQ is carbon dioxide, a natural byproduct of human and animal respiration; decaying organic matter; and combustion of wood, carbohydrates and fossil fuels. At low densities, CO2 is odorless and tasteless. However, its differential indoor concentration is a surrogate for certain air-quality metrics, particularly occupant perception of odorous bioeffluents (body odor). Inadequate building ventilation promotes excess moisture and mold. Elevated CO2 increases complaints of stale air while impairing productivity and decision-making.
According to the Lawrence Berkeley National Laboratory, “On nine scales of decision-making performance, test subjects showed significant reductions on six of the scales at CO2 levels of 1,000 ppm and large reductions on seven of the scales at 2,500 ppm. The most dramatic declines in performance, in which subjects were rated as ‘dysfunctional,’ involved taking initiatives and thinking strategically.”
This research challenges conventional wisdom that CO2 concentrations of 5,000 ppm are acceptable occupational limits in the work environment.
Global building standards increasingly dictate tight building envelopes that purposely restrict outside-air infiltration to conserve energy and reduce carbon footprint. Ironically, these well-intentioned conservation measures compete with the need to reduce indoor-air pollutants, including volatile organic compounds (VOCs), carbon monoxide (CO) and CO2 buildup from anthropogenic sources. Moreover, existing ventilation standards are typically minimum recommendations and may not result in optimal air quality.
Modern structures rely on adaptive demand-controlled ventilation (DCV) to modulate air exchange according to real-time measurement of CO2 concentration, a proxy for occupant load. When properly deployed and calibrated, auxiliary CO2 loggers throughout a DCV facility ensure the ventilation system works as intended, while identifying potential duct blockages or control-system issues.
Most facilities, however, rely on fixed mechanical systems and natural ventilation, blind to dynamic occupancy levels and other environmental factors. At certain times of the year when inside-outside temperature differentials are great, windows may be shut to conserve energy, promoting high CO2 concentration and trapping unhealthful indoor pollutants.
As a result, marginal or poor indoor-air quality is commonplace. Studies from around the world consistently document elevated indoor CO2 concentrations ranging from under 1,000 ppm to extremes of more than 6,000 ppm.
When assessing indoor levels, it is important not only to measure absolute CO2 ratios but also to compare them to outdoor levels. Ventilation effectiveness relates to the difference between indoor and outdoor levels, whereas health considerations and cognitive function correlate both to overall ventilation, as well as the absolute CO2 levels present in a structure.
Differences in data loggers
CO2 data loggers detect and manage the risk of elevated CO2, which is essential to an abatement strategy that may include:
- Improved mechanical ventilation and air flow
- Injection of fresh outdoor air via energy-efficient heat exchangers
- Judicious use of operable windows
- Ceiling fans to promote better air circulation
- Reduction or mitigation of internal CO2-producing sources
Still, choosing the right logger can be a daunting challenge without an understanding of the key differentiators among available devices. Although many loggers offer comparable measurement ranges and sensor accuracies, other factors primarily related to usability include connectivity options, alarming, battery life and long-term calibration costs, which can vary more widely.
Smart wireless technology allows for efficient data collection, ease in data sharing and user-friendly operation. Data loggers can incorporate Bluetooth Low Energy (BLE) technology to retrieve data and logger status using phones and tablets.
A BLE-equipped CO2 data logger records and transmits wirelessly to mobile devices on demand, enabling measurements from the logger remotely up to a 100-foot range (line-of-sight). Eliminating the requirement to log on to the Internet, pair devices, install computer software, or connect the logger to a computer for downloading data reduce time and costs associated with CO2 monitoring programs.
BLE solutions are particularly advantageous when deploying multiple CO2 data loggers inside buildings, or in hard-to-reach locations where physically downloading data from the logger would otherwise prove difficult. Ventilation-system monitoring is one example where loggers may reside out-of-reach or inside a return-air duct.
A CO2 data logger should have an LCD display with programmable alarm notifications to display current CO2 levels, logging status, battery use, memory consumption, and other parameters such as ambient temperature and relative humidity. With user-programmable alarm notifications, a CO2 data logger issues an audible alert and displays a visual warning on its LCD screen during a qualified trigger event, such as when a CO2 concentration exceeds a healthy threshold. Users should look for options that provide both audible and display-based alarm notifications for alerts to problems as they occur to aid efficient corrective action.
Another quality to look for is long battery life. Many loggers run on battery only for short periods or require an expensive proprietary battery, yet other battery options allow up to six months of continuous logging at five-minute intervals using standard alkaline or lithium AA batteries. With extended battery life users gain flexibility; increased spatial coverage; and an ability to set up CO2 monitoring in locations where no AC power exists, such as return-air ducts in or near HVAC.
USB ports can also improve flexibility of data access and analysis. Through a port, users can connect a CO2 data logger directly to a computer running graphing and analysis software, enabling quick plotting, comparison and data extraction. This information visually reinforces presentations and recommendations with a compelling engineering foundation.
The ports allow greater power versatility, supporting a wider range of application scenarios, including using off-the-shelf USB chargers to power data loggers for longer deployments. When faster sampling rates/alarm responses are required, the USB port is a convenient remote-power option for uninterrupted 24/7 operation when using a recirculating memory cache.
Ease of calibration
A logger should have manual and automatic calibration, as well as elevation/altitude compensation. Most loggers require periodic calibration to a known reference and careful placement to avoid erroneous results. Some advanced devices include automatic temperature and dynamic pressure compensation, but this is less common on mid-range products.
Many commercial CO2 loggers, including the Onset MX1102 CO2 Logger with BLE technology, use maintenance-free non-dispersive infrared (NDIR) sensing technology that features a gas chamber, infrared (IR) source transmitter, optical filter and IR detector. The detection wavelength is tuned to measure the concentration of CO2 molecules to a reasonable degree of precision. Depending on sensor design, the transmitter and detector can drift over time, causing long-term errors and under-reporting of actual CO2 concentrations. While optimal calibration methods require calibrated gas concentrations and sealed test chambers, this is not cost effective or practical for maintenance personnel, particularly where a large number of devices may be operating.
In lieu of calibrating with specialized gases, engineers have developed manual and automated calibration algorithms to improve accuracy for typical CO2 logging applications, while keeping operating and maintenance costs to a minimum. (For more on calibration issues, click here.)
Carbon dioxide is more than 60 percent denser than air and tends to settle, which can lead to distorted readings if the logger is placed too low on a wall. Over time, however, diffusion occurs, and CO2 concentrations should be near the same across the total volume of air.
For typical IAQ measurements, mount the logger in the breathing zone approximately 48 to 72 inches vertically above the floor surface and several feet away from anyone whose exhalation could alter localized CO2 concentration. Stay at least 36 inches away from any corner; 24 inches from an open doorway; and well away from operable windows, outside doors and vents.
Some manufacturers recommend mounting the loggers inside the return-air duct in the rooms of interest, assuming such a duct exists.
CO2 concentration is a key indicator of indoor air quality and ventilation effectiveness. CO2 levels more than 600 to 700 ppm above outdoor levels warrant special focus on ventilation function and emission sources. Although many existing global standards stipulate maximum daily average exposure limits up to 5,000 ppm, research shows that cognitive impairment and perception of poor air quality commence at 1,000 ppm (absolute) or 600 to 700 ppm (differential) above outside levels. Keeping CO2 levels in check typically helps reduce other pollutants due to engineering focus on improved ventilation.
Facility managers and building engineers have a responsibility to ensure compliance with all laws. They also are committed to recommended best practices, which frequently go well beyond minimum requirements set forth in building codes and ventilation standards.
When selecting a CO2 logger, evaluate overall specifications as well as wireless compatibility with Bluetooth-enabled mobile devices, integrated display and alarms, long battery life, flexible USB connectivity, and automatic and manual calibration modes. Quality CO2 data loggers provide a cost-effective method to assess indoor CO2 concentration levels, helping to eliminate sick-building syndrome and harmful pollutants typical of tight and poorly ventilated structures.
Editor’s note: For a full list of references used in the preparation of this article, a PDF of the original application note is available for download from buildera.com.
Greg Lowitz is the founder and CEO of Buildera, a global provider of structural forensics systems, environmental data logging, and product evaluation services for professionals and property owners. He holds bachelor’s and master’s degrees in electrical engineering from Stanford University.
Based on Cape Cod, Massachusetts, Onset designs and manufactures its HOBO data loggers on site. HOBO data logger products are used around the world in a range of monitoring applications, from verifying the performance of green buildings and renewable energy systems to agricultural and coastal research.