Carbon Dioxide: An Invisible Indoor Air Pollutant?

Carbon dioxide (CO2) is a gas that is present in both indoor and outdoor air. Indoors, CO2 is primarily produced by human respiration, but it can also come from other sources such as combustion by-products and outdoor air ventilation. While it is not usually found at hazardous levels in indoor environments, elevated concentrations of CO2 can have adverse effects on human health and performance. The presence of CO2 in indoor air has been linked to poor air quality, acute health symptoms such as headaches and mucosal irritation, slower work performance, and increased absence. These effects are believed to be correlated with higher levels of other indoor-generated pollutants that directly cause adverse effects. As a result, maintaining good ventilation and adhering to ventilation guidelines are crucial to ensuring indoor air quality and minimizing the potential impacts of CO2 on human health and decision-making abilities.

CO2 is mainly produced indoors through human respiration

Carbon dioxide (CO2) is a natural byproduct of human respiration. As a result, indoor spaces with high occupancy rates tend to have higher concentrations of CO2 than outdoor environments. The more people occupying an indoor space, the higher the levels of CO2. The length of time an area has been occupied also impacts the concentration of CO2.

The indoor CO2 concentration can be lowered by increasing the amount of outdoor air ventilation. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has developed guidelines to maintain comfortable indoor air quality for occupants. For example, in office spaces, ASHRAE recommends 17 cubic feet per minute of outdoor air per person (for a 1,000 square foot space occupied by 5 people). This rate of ventilation should keep carbon dioxide concentrations below 1,000 ppm, which is considered acceptable indoor air quality for most people.

However, in areas with many occupants and limited outdoor air ventilation, indoor CO2 concentrations can exceed 1,000 ppm. For example, in classrooms with 35 students, the CO2 levels can rise above 2,000 ppm. At these levels, the air flow on the furnace should be adjusted to bring in more fresh air. If the carbon dioxide level rises above 2,000 ppm, this can be a serious issue that may require modifications to the HVAC system.

While CO2 itself is not typically found at hazardous levels in indoor environments, high concentrations can be associated with poor air quality and adverse health effects. Studies have shown that higher levels of CO2 indoors are linked to perceptions of poor air quality, increased prevalence of acute health symptoms such as headaches and mucosal irritation, slower work performance, and increased absence. However, it is important to note that these effects may be due to other indoor-generated pollutants that correlate with higher CO2 concentrations, rather than the CO2 itself.

CO2 concentrations indoors are higher than outdoors

Carbon dioxide (CO2) concentrations inside buildings are often much higher than those outdoors. This is due to the fact that humans produce and exhale CO2 as part of their normal metabolic processes. As a result, the indoor CO2 concentration increases as the ventilation rate (the rate of outdoor air supply to the indoors) per person decreases.

The indoor CO2 concentration can be used as a rough indicator of the outdoor ventilation rate per occupant. For example, if the outdoor air supply is insufficient, the indoor CO2 concentration will be higher. Organizations like the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) provide guidelines and data on the importance of monitoring CO2 levels indoors and the potential long-term effects of exposure.

The acceptable indoor CO2 concentration depends on various factors, including the type of facility and room. For instance, ASHRAE recommends 15 cubic feet per minute per person of outdoor air for elementary school classrooms, while for office spaces, it recommends 17 cubic feet per minute per person. Maintaining CO2 levels below the recommended thresholds ensures better ventilation and a higher level of comfort for occupants.

According to Persily and Gorfain (2008), typical outdoor CO2 concentrations are approximately 380-500 ppm, while indoor concentrations can range from outdoor levels up to several thousand ppm. However, it is important to note that CO2 levels alone do not determine overall indoor air quality. Other factors, such as temperature, humidity, noise, ventilation, human bio-effluents, lighting, and indoor air pollutants, also play a role in indoor air quality.

CO2 levels can be controlled through ventilation

Carbon dioxide (CO2) is a harmful gas when present in confined spaces and in elevated concentrations. As humans produce and exhale CO2, concentrations of this gas in occupied indoor spaces are higher than outdoors. The magnitude of the indoor-outdoor difference in CO2 concentration increases as the ventilation rate per person decreases.

Elevated levels of CO2 in indoor spaces, especially over an extended period, can lead to a variety of health and wellbeing issues. These include perceptions of poor air quality, increased prevalence of acute health symptoms (e.g. headaches, mucosal irritation), slower work performance, and increased absence.

CO2 sensors can be used to accurately measure the concentration of CO2 in the atmosphere, with a higher detected level indicating a larger number of people being present. By analyzing the recorded sensor data, the ventilation system can be automatically adjusted to reflect employee numbers. This is called Demand Control Ventilation (DCV) and combines sensors, the Building Management System (BMS), and intelligent ventilation management to deliver optimized air flows. The DCV adjusts the amount of outside air that is introduced into the building to reduce the CO2 levels, providing optimal air control and cost control.

To assess for adequate ventilation, or the removal of CO2, one can check the patient's arterial carbon dioxide levels (PaCO2) on an arterial blood gas (ABG). Then, the minute ventilation can be adjusted by increasing or decreasing the tidal volume (VT) or respiratory rate (RR) on the ventilator.

CO2 at 3000 ppm, alongside moderate bio-effluent levels, is harmful

Carbon dioxide (CO2) is a colourless, odourless gas produced both naturally and through human activities, such as burning gasoline, coal, oil, and wood. It is also exhaled by people, contributing to CO2 levels in the air. The outdoor concentration of carbon dioxide is about 400 parts per million (ppm) or higher in areas with high traffic or industrial activity.

Indoors, carbon dioxide concentrations can vary from several hundred ppm to over 1000 ppm in areas with many occupants for extended periods and limited outdoor air ventilation. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has set ventilation guidelines to maintain comfortable indoor environments, recommending a minimum of 15-17 cubic feet per minute of outdoor air per person in spaces such as classrooms and offices. These guidelines aim to keep CO2 levels below 1000 ppm, creating acceptable indoor air quality for most individuals.

While CO2 itself is not typically found at hazardous levels indoors, elevated concentrations can indicate insufficient ventilation, leading to a build-up of other contaminants. At levels above 5000 ppm and with prolonged exposure, CO2 can cause adverse effects such as headaches, dizziness, and nausea. Even higher levels, around 40,000 ppm, can lead to asphyxiation and are immediately dangerous to life and health.

Therefore, maintaining proper ventilation and indoor air quality is crucial. While the general workplace safety standards set by the Minnesota Department of Labor and Industry (MNDOLI) allow for concentrations of up to 10,000 ppm over an 8-hour period, sensitive populations like children and the elderly may experience health effects at lower levels.

Given this context, CO2 levels at 3000 ppm, even with moderate bio-effluent levels, are not likely to be harmful. Bio-effluent refers to treated wastewater that flows out of treatment plants, sewers, or industrial outfalls and can contain pollutants such as heavy metals, chemicals, and other contaminants. While it is essential to manage bio-effluent through proper treatment and adherence to regulations, the primary concern with indoor air quality is the CO2 levels and adequate ventilation to prevent the build-up of other pollutants.

CO2 at 5000 ppm for many hours can cause dizziness and nausea

Carbon dioxide (CO2) is a known indoor pollutant. Indoor carbon dioxide concentrations are higher than those outdoors, as humans produce and exhale CO2. As the ventilation rate decreases, the magnitude of the indoor-outdoor difference in CO2 concentration increases.

The normal level of CO2 in fresh air is approximately 400 ppm (parts per million) or 0.04% CO2 in air by volume. However, carbon dioxide concentrations indoors can vary from several hundred ppm to over 1000 ppm in areas with many occupants for an extended period and where outdoor air ventilation is limited.

In indoor settings, a CO2 concentration of 400-1000 ppm is considered acceptable. In office spaces and classrooms, a common guideline is to maintain CO2 levels below 800-1000 ppm. Higher CO2 levels have been linked to decreased cognitive performance and reduced productivity.

At very high levels, CO2 itself can cause dizziness and nausea. The American Conference of Governmental Industrial Hygienists (ACGIH) recommends an 8-hour Threshold Limit Value (TLV) of 5000 ppm. CO2 at 5000 ppm for many hours can cause dizziness and nausea. While this is considered a safe level, it is at the upper limit of what is typically found in buildings.

In extreme cases of poor indoor air quality with very high CO2 levels, some people may experience nausea, dizziness, or other discomforts. It is important to identify the health effects associated with exposure to low-level CO2 concentrations, as modern populations in developed countries spend approximately 90% of their time indoors.

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