Carbon Monoxide in Scuba Diving: Risks, Sources, and How to Protect Yourself and Team

• By LMiller
• Posted in Uncategorized
• On March 1, 2026
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Introduction

Carbon monoxide (CO) contamination in scuba cylinders is uncommon, but when it occurs, the consequences can be severe. CO interferes with the body’s ability to transport oxygen, and even small amounts become more dangerous as a diver descends. This article explains how CO affects the body, how contamination occurs, why depth amplifies the risk, and what divers can do to protect themselves.

What Carbon Monoxide Is and Why It Matters

Carbon monoxide is a colorless, odorless, and tasteless gas produced naturally by forest fires, volcanic activity, and atmospheric reactions. It is also generated by burning fuels and internal combustion engines.

For divers, the concern is not environmental CO but contamination inside scuba cylinders. CO can enter a cylinder through a malfunctioning compressor, poor intake placement, or thermal breakdown of compressor oils. Once inhaled, CO interferes with oxygen transport in a way that can quickly become life‑threatening underwater.

How CO Disrupts Oxygen Transport

Hemoglobin is a protein inside red blood cells that carries oxygen to the body’s tissues. Each hemoglobin molecule can bind up to four oxygen molecules, and the binding is cooperative — once one oxygen binds, the next binds more easily.

CO disrupts this process in two ways:

• It competes directly with oxygen for the same binding sites and attaches with more than 200 times the affinity.
• It prevents oxygen release, locking hemoglobin into a form that holds onto any remaining oxygen instead of delivering it to tissues.

The result is a rapid decline in oxygen delivery to vital organs. Symptoms may appear gradually or suddenly, depending on concentration and exertion.

Figure 1 shows a simplified version of the iron-containing heme group of hemoglobin.  The molecule on the left shows a bound O2 molecule resulting in an approximate 121° angle between the oxygen atoms, while CO binds in a straight 180° (right side).  The difference in the stereochemistry is believed to contribute to the higher affinity of CO.

How Much CO Is in Clean Air?

Carbon monoxide is naturally occurring in the environment, but at very low levels.  As divers, we know that dehydrated air is composed of approximately 20.9% oxygen (O2), 78.1% nitrogen (N2), 0.9% argon (Ar), 0.05% carbon dioxide (CO2) and trace amounts of other elements including CO.  The level of CO is small enough that expressing as a percent is cumbersome.  For this reason, it is expressed as parts per million (ppm).  Parts per million is a ratio of one part of solute in 1 million parts of total substance.  Clean outdoor air contains about 0.1 ppm of CO — roughly 0.00001%. This amount is harmless.

Regulatory exposure limits include:

• EPA: 9 ppm averaged over 8 hours
• OSHA: 50 ppm over an 8‑hour workday

Tables 1 summarize the health effects of CO exposure at increasing concentrations. These values matter to divers because symptoms at low levels — headache, dizziness, nausea — can easily be mistaken for seasickness, dehydration, or fatigue.

Table 1: Health Implications Due to Increasing Levels of CO Exposure

CO Level (ppm)	Description Health Implications
0.1	Natural atmospheric level in clean outdoor air. No health effects for humans or animals.
1	0.96% increase in hospitalization for cardiovascular issues. Potential risk for elderly individuals with heart conditions.
3-7	6% increase in asthma-related hospital admissions. Increased asthma symptoms, especially in sensitive individuals.
5-6	Risk of low birth weight in prolonged exposure during pregnancy. Potential developmental effects on fetuses.
9	Maximum allowable outdoor air level (EPA & WHO). Can affect individuals with pre-existing health conditions.
50	OSHA max workplace exposure limit for an 8-hour period.
100	Symptoms like headache, dizziness, tiredness within 2 hrs.
200	Severe symptoms, nausea, confusion; OSHA & NIOSH evacuation recommended.
400	Headache within 1-2 hours; life-threatening after 3 hours
800	Unconscious within 2 hours; death possible in 2-3 hours.
1,600	Symptoms appear in 20 minutes; death within 1 hour.
12,800	Unconscious after 2-3 breaths; death occurs in under 3 minutes.

Interpreting CO Exposure Tables

For healthy individuals, CO levels below 9 ppm are generally not dangerous. However:

• 50–100 ppm can cause headache, dizziness, and nausea within hours.
• 200–800 ppm can lead to confusion, loss of coordination, unconsciousness, and death with short exposure.

These symptoms overlap with common non-diving and diving‑related conditions, making CO contamination easy to miss until it becomes severe.

Where CO Contamination Comes From

Regulatory Gaps

Recreational fill stations are not legally required to test for CO. As guidance, the Compressed Gas Association’s (CGA) Grade E standards for scuba diving fills is ≤ 10 ppm.  Many reputable operators follow industry standards or exceed them, but the responsibility ultimately falls on the diver to assess fill‑station quality and test their own gas.   

Assessing Fill Stations

A clean, organized fill station with proper intake placement and documented maintenance is far less likely to produce contaminated gas. Conversely, a cluttered, poorly maintained setup increases the risk of CO contamination.

When traveling or diving in remote areas, visual inspection and asking questions about compressor maintenance become especially important.

Why CO Becomes More Dangerous at Depth

Dalton’s Law states that the partial pressure of each gas increases proportionally with depth. This means that even low CO levels at the surface become dangerous underwater.

For example:

• 50 ppm at the surface becomes ~200 ppm at 100 fsw
• 400 ppm at the surface becomes ~1,600 ppm at 100 fsw, enough to cause severe symptoms within minutes

As partial pressure increases, CO binds to hemoglobin more rapidly, accelerating oxygen deprivation.

Testing for CO Contamination

Divers can eliminate the risk of CO contamination by testing each fill. Important considerations:

• Home CO alarms are not suitable — they typically detect only above ~70 ppm and require long exposure times.
• Scuba‑specific CO analyzers detect low levels quickly and are designed for compressed‑gas testing.
• Sensors must be replaced regularly (e.g., every 2 years) according to manufacturer recommendations.

Table 2 lists several analyzers available or use by divers.

Brand	Model	Cost	Lower Limit	Link
Safety Lab Plus, Inc	Pro-CO (disposable)	TBD	5 ppm	https://co-pro.com/
Forensic Detectors	Carbon Monoxide Meter	$89.00 (analyzer) $55.00 (replacement sensor)	0 ppm	https://www.forensicsdetectors.com/products/co-meter-0-1ppm
Forensic Detectors	Carbon Monoxide Super-Meter	$185.00 (analyzer) $55.00 (replacement sensor)	0 ppm	https://www.forensicsdetectors.com/products/co-meter-0-1ppm
Palm	PALM-CO Carbon Monoxide Analyzer	$600.00 (analyzer) $276.00 (replacement sensor)	1 ppm	Palm-CO Carbon Monoxide Analyzer | Dive Right In Scuba   https://www.divegearexpress.com/aii-osv-22-af-carbon-monoxide-sensor-palm-co

Receiving a Positive Test

If a cylinder tests positive for CO:

• Do not use the gas.
• Notify the fill station immediately.
• Retest with a second analyzer if possible.
• Drain and refill the cylinder from a reputable source.

There is no need to clean or VIP the cylinder solely due to CO contamination.

Recognizing CO Exposure During a Dive

Early symptoms include:

• headache
• nausea
• dizziness
• unusual fatigue
• confusion or impaired judgment

These symptoms worsen with exertion because with strenuous activity the tissues require more oxygen. Multiple divers may be affected if they received fills from the same contaminated source.

Emergency Response

If CO exposure is suspected:

• Abort the dive and begin a controlled ascent.
• If decompression is required, switch to an uncontaminated gas source if available (e.g., alternative gas or buddy breath).
• Consider the severity of symptoms and determine if a safety stop is appropriate
• Once on the surface, rest, breathe 100% oxygen if available, and seek medical evaluation.
• Secure the suspected cylinder for testing.

Symptom Overlap With Other Conditions

Unfortunately, there is overlap across CO exposure and other diving and non-diving related health conditions.  Early warning signs of CO exposure could be dismissed as being due to sea sickness, travel, or a poor night’s sleep.  For this reason, it is important to consider the entire situation when attempting a diagnosis (diver’s health, type of dive, weather and water conditions, etc.).

Headache

• CO exposure
• seasickness
• heat illness
• decompression illness (sometimes)

Nausea

• CO exposure
• seasickness
• heart attack
• heat illness

Confusion or Cognitive Changes

• CO exposure
• narcosis
• stroke
• severe heat illness
• severe decompression illness

Conclusion

CO contamination is not a constant, systemic threat lurking in every scuba cylinder. It is a context‑dependent hazard that becomes dangerous only when specific failures or environmental conditions occur. Because these failures are unpredictable and the consequences are potentially fatal, divers benefit from a mindset of situational awareness, instead of fear.  Because recreational fill stations are not required to test for CO, divers must take responsibility for their own safety.  Older, remote, or makeshift operations deserve more scrutiny than established, well‑maintained shops.  Testing each fill, especially when traveling or using unfamiliar operators, is a simple and effective way to eliminate a life‑threatening hazard.

Do you routinely test for CO contamination after a fill? Have you experienced a CO contamination situation? Leave your comments below.

References and Additional Reading

• DAN Article – Carbon Monoxide Detection Devices for Divers –
• DAN Article – Carbon Monoxide Safety
• DAN Article – Carbon Monoxide the Silent Killer
• https://www.hseblog.com/carbon-monoxide-levels/
• https://www.cdc.gov/carbon-monoxide/hcp/clinical-guidance/index.html
• https://handbook.cganet.com/
• Report on CO-related Fatalities