Safe CO Levels in Mining: MSHA's 50 ppm Limit Explained

Carbon monoxide levels in mining operations pose a serious risk because the industry uses various combustion-based equipment. MSHA sets the safe carbon monoxide limit in mining at 50 ppm averaged over an 8-hour shift, with a lower short-term limit of 75 ppm for 15 minutes in coal mines. Levels above 200 ppm can cause confusion and loss of coordination, which is why continuous CO monitoring matters.
To manage CO safely, companies should understand permissible exposure limits, when CO levels pose a danger, and factors that may increase the likelihood of CO exposure (such as inadequate ventilation and CO emissions from diesel engines). Therefore, companies should develop effective CO safety programs that include well-defined responses to CO incidents, continuous monitoring for elevated CO levels, and proactive alarm thresholds. A real-time approach to CO monitoring protects miners, ensures compliance, and maintains a safe mining environment.
 

Carbon monoxide is one of the most dangerous yet overlooked hazards in mining environments, posing serious health risks to miners. From diesel-powered machinery and blasting operations to ventilation challenges, both underground and surface mines regularly face CO-related risks. Understanding safe limits, dangerous thresholds, and risk management is crucial for safety. This blog explains why carbon monoxide poses a serious threat in mining, outlines permissible exposure limits, identifies key risk factors, and explores how proper monitoring and response strategies can prevent CO-related incidents.

One of the most significant hazards in mine environments, particularly underground mining sites, is the increased risk of carbon monoxide (CO) poisoning from machinery and blasting (e.g., diesel engines, explosives).
CO exposure risks exist in both underground and open-pit mines. Diesel engines produce less CO than gasoline engines, but in confined or low-airflow areas, even small amounts of exhaust can build up into CO pockets.
CO's health effects are dire, and the risks of CO exposure are well established. CO binds to haemoglobin about 200-250 times more strongly than O2 does. The U.S. Mine Safety and Health Administration (MSHA) recognises that CO levels above 50 ppm averaged over 8 hours are potentially hazardous (NIOSH recommends a stricter 35 ppm); however, short-term exposure to higher concentrations (e.g., 400 ppm for 3 to 5 hours) can be fatal. The health symptoms resulting from CO exposure may include, but are not limited to, headaches, unconsciousness, and death at extremely high levels.
 

Identifying safe carbon monoxide (CO) levels in the mining industry is critical to protecting workers' health and safety. MSHA's 8-hour exposure limit for carbon monoxide is 50 parts per million (ppm). Short-term limits differ by mine type: coal mines are held to 75 ppm for 15 minutes, while metal and nonmetal mines use a 400 ppm, 15-minute limit. These limits protect against both acute and chronic effects of CO exposure.
Because ventilation, fuel sources, and other mine-specific factors vary, many mining companies take a more conservative approach to setting a safe operational CO level. These operational targets (25–30 ppm) offer an additional safety margin against ventilation failures.
When a mine experiences a short-term spike in CO levels above 50 ppm, the mine operator takes immediate action by adjusting ventilation, instituting an evacuation plan, or shutting down equipment. Incorporating regulatory guidelines and operational goals provides a safety net for all employees.
 

Limits vary by country and regulator, so check the rules that apply to your site.
 

MSHA applies different short-term CO limits depending on mine type.

For diesel exhaust CO in coal mines, MSHA also lists a 25 ppm action level. Source: MSHA Health Hazard Cards HH-28 and HH-27. 

When Carbon Monoxide (CO) levels become high in the mine working environment - even for short durations- it can become a health hazard.  Although many regulations dictate that 50 parts per million (ppm) of CO would be a permissible level for an eight-hour day of mining operations, CO levels may adversely affect miners even if they are below these levels if the miners are exposed to CO for extended periods of time and/or if ventilation is poor.

CO exposure is more dangerous above 200 ppm; miners may become confused and experience coordination problems. Many incidents illustrate the dangers of excessive CO exposure. In November 2013, MSHA's preliminary accident report for the Revenue silver mine described a miner entering an area where explosives had recently been detonated; rescue teams detected fatal levels of carbon monoxide during the recovery operation. In India, experts assessing gas leaks from underground coal mines in Dhanbad, Jharkhand recorded CO at up to 1,500 ppm in December 2025, around 30 times the permissible limit, according to PTI.

Even frequent exposure to moderate CO levels can cause long-term neurological and cardiovascular effects. Continuous monitoring, proper ventilation, and strict compliance with both regulatory and operational limits are therefore essential to prevent CO from becoming a silent but deadly threat in mining operations.
 

Carbon monoxide (CO) poisoning can pose a danger to workers in underground mines due to the varied operational and environmental conditions that create a high likelihood of CO buildup. These factors can be used to develop safety procedures for mining operations; however, understanding them is essential before designing safety systems.

1. Equipment Powered by Diesel and Explosive Use

All equipment that uses diesel fuel (i.e., diesel generators, diesel engines, etc.), and all blasting operations generate CO through the combustion of diesel fuel. Thus, the number of diesel-powered machines operating at any one time increases the potential for CO accumulation, particularly in confined areas with limited air exchange.

2. Ventilation

The design of underground mines consists of passages/tunnels that restrict air exchange, creating a much greater vulnerability to CO poisoning. Poorly designed or maintained ventilation systems in underground mines can increase the potential for rapid CO accumulation.

3. Mine Configuration and Depth

Long, winding tunnels and deep shafts are both environments with minimal natural air movement (i.e., fresh air).

4. Environmental Factors

Environmental factors, including: Temperature Inversion (i.e., a warm air layer forms above a cooler air layer); Humidity (high); and Wind Speed (low), may cause pockets of CO to build up in surface mining operations.

5. Operational Procedures

Continuous use of machines without monitoring their emissions, lack of routine maintenance, insufficiently scheduled blasting, and/or engine operations could further increase the risk of CO poisoning.

By recognizing these contributing factors, mining operations can prioritize ventilation improvements, equipment maintenance, and proactive monitoring to reduce the likelihood of dangerous CO exposure.

In mining operations, effective monitoring of carbon monoxide is essential to protect miners and comply with regulations. To provide a safe working environment, mining operations can combine continuous and periodic monitoring.

Mining sites often use fixed CO gas monitors for worker safety, while larger outdoor areas or surface mining zones may use systems like CAAQMS to track broader ambient air conditions. Continuous monitoring systems are most useful in high-risk work areas, such as underground tunnels, engine rooms, and blasting areas. Periodic monitoring can be conducted using handheld detectors or through manual sampling.

For practical purposes, continuous carbon monoxide monitoring should be conducted near potential carbon monoxide emission sources, work areas, and at points with inadequate ventilation. Calibration and maintenance of continuous monitoring equipment must be performed regularly to ensure reliability. Additionally, alarm thresholds for continuous monitoring systems should be set below the regulatory threshold.

Oizom's AQBot is an industrial single-parameter monitor suited to continuous fixed monitoring at high-risk points such as tunnels, engine rooms, and blasting areas, while the portable Pollusense tracks up to 9 toxic gases for spot checks and site audits.
 

When carbon monoxide (CO) concentrations at a mine site exceed established permissible levels, you must act quickly. First, evacuate everyone in the area while gathering information on the CO level, hazards, and the need to prevent further exposure.
Once evacuation is complete, identify and control CO sources. The mine’s ventilation systems should be increased to dilute the accumulated CO concentration. CO monitoring systems should be monitored continuously to ensure CO concentrations are declining. Operations should resume only once CO concentrations are consistently well below established permitted and operational levels.
Corrective actions may include conducting a root cause analysis, inspecting equipment for evidence of incomplete combustion, evaluating the ventilation system's effectiveness, and reviewing alarm setpoints.
 

By its nature, periodic CO monitoring provides only a snapshot of air quality over a given interval. Sudden spikes in the levels of CO can occur with the use of equipment, blasting, and failure of ventilation systems.

Continuous CO monitoring allows for immediate identification when CO concentrations begin to rise. Rather than having to react after the fact, such as after a worker has been exposed to high levels of CO, mining operations will be able to take action to address the problem before it creates a dangerous situation for the workforce, either by making changes to ventilation systems, shutting off equipment, and/or evacuating areas that may be affected by CO exposure.

Commercialized monitoring systems, such as those produced by Oizom, can automatically alert to issues and centrally store data, giving mining companies a tool that helps create a safe work environment, comply with regulations, and provide more data for decision-making.

A proactive rather than reactive approach is required to manage carbon monoxide hazards in mining. It is vital to promptly detect carbon monoxide in the workplace. While exposure limits and safety procedures have been established, they do not provide the same level of visibility as ongoing monitoring. Mining companies that use solutions that enable ongoing monitoring of carbon monoxide concentrations can respond to hazards more quickly, maintain regulatory compliance, and prevent their workers from being exposed to the dangers associated with carbon monoxide. Thus, regular monitoring of carbon monoxide is an essential component of a safe modern mining work environment.