Carbon monoxide explained — how it forms, what it does, and the limits at work

November is carbon monoxide month at JMS. Later this month, UK Carbon Monoxide Awareness Week will focus on CO in homes. Over the next four weeks we're looking at it at work: what it is, where it comes from, and how to read what your gas monitor is telling you. This first guide covers the basics.

What carbon monoxide is

Carbon monoxide (CO) is a gas made of one carbon atom and one oxygen atom. It has no colour, no smell and no taste, and it doesn't irritate the eyes or throat. Nobody can tell it is there without a detector.

Its density is almost exactly that of air (a relative density of about 0.97), so it mixes into the surrounding air rather than collecting at a particular height. Where it ends up depends mostly on temperature and on what it is mixed with.

CO is also flammable, but only at around 10.9% by volume. At work, its toxicity is what matters: it causes harm at concentrations thousands of times lower.

How it forms: incomplete combustion

Any fuel containing carbon (natural gas, LPG, petrol, diesel, oil, coal, coke, wood) produces carbon dioxide and water when it burns with enough oxygen. That is complete combustion.

When there isn't enough oxygen, the carbon is only partly oxidised and carbon monoxide forms instead. That is incomplete combustion. Common causes include:

• too little air reaching the flame, for example in an enclosed or poorly ventilated space
• a blocked or damaged flue, or exhaust that can't escape
• a flame cooled by contact with a cold surface
• poorly adjusted or poorly maintained burners and engines

Some processes produce CO without any open flame. Steelmaking generates gases that are largely CO, and stored wood pellets slowly give it off as they oxidise. We cover these in our next guide on where CO turns up at work.

What it does in the body

Oxygen travels round the body attached to haemoglobin in the red blood cells. Carbon monoxide binds to haemoglobin around 200 to 250 times more readily than oxygen, forming carboxyhaemoglobin. Every site it occupies is one that can't carry oxygen.

It also makes the remaining haemoglobin hold on to its oxygen more tightly, so less is released where it is needed. The brain and heart, which use the most oxygen, are affected first.

Common early symptoms are:

• headache
• dizziness
• nausea
• tiredness and breathlessness
• confusion

These are easy to mistake for a cold, flu or a long day. At higher exposures, CO causes loss of consciousness and can be fatal. Significant exposures can also leave lasting effects on memory and concentration.

CO leaves the body slowly. In fresh air it takes several hours for the level in the blood to halve, so exposure can build up over repeated entries during a shift. Some people are more vulnerable, including those with heart conditions and pregnant workers. Smokers already carry some carboxyhaemoglobin before they start work.

CO levels and their effects

The table below is a widely published guide to how a healthy adult at rest responds to different concentrations. It is indicative only: effects vary between individuals and come on faster with physical work, which increases breathing rate.

UK workplace exposure limits

In Great Britain, workplace exposure limits (WELs) are set out in HSE's EH40 and apply under the COSHH Regulations. For carbon monoxide they are:

• 20 ppm as an 8-hour time-weighted average (TWA)
• 100 ppm as a 15-minute short-term exposure limit (STEL)

Since 21 August 2023 these limits apply in every sector.

A TWA is the average exposure across a working day. Short periods above 20 ppm can be balanced by time well below it, as long as the average stays within the limit. A STEL covers short peaks, averaged over any 15-minute period.

Most personal gas monitors track all three numbers: the live reading on the display, a rolling STEL and a running TWA. It is worth knowing where to find each on your own instrument, and which one an alarm relates to.

Two points help put the limits in context. They are legal maximums for personal exposure, and COSHH still requires exposure to be kept as low as reasonably practicable. And in a confined space, whether an atmosphere is acceptable for entry is decided by the risk assessment and permit, not by the WEL alone.

If your team would benefit from a refresher on reading TWA and STEL values, the JMS competency assessment is a quick way to see where understanding is strong and where it isn't.

Detecting carbon monoxide

Because CO can't be seen, smelt or tasted, a detector is the only reliable way to know it is there. Personal gas monitors normally use an electrochemical sensor for CO.

These sensors are accurate and reliable, but many also respond to some other gases, hydrogen in particular. We looked at this in October. If a CO reading appears with no obvious source, it is worth investigating rather than dismissing.

As with any sensor, a CO channel is only as good as its maintenance: regular bump testing, calibration in line with the manufacturer's instructions, and people who understand what the readings mean.

How we can help

For more than 30 years we've helped people understand the atmospheres they work in and the instruments that measure them.

Our GDU course is for people who wear gas monitors on the job. AGT trains authorised gas testers. Both run as open courses or at your site. If you have questions about carbon monoxide on your site, get in touch.

Next in the series: where carbon monoxide turns up at work.