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Detecting Low Level Carbon Monoxide (CO)

There is no truly harmless CO concentration level, even small concentrations have an impact over time. Inhaled, carbon monoxide is absorbed into the bloodstream and binds mainly with haemoglobin (part of red blood cells) and forms carboxyhaemoglobin which results in a reduction of the amount of oxygen that can be carried around the body.

An increase of long-term low level carbon monoxide concentration in the air (described usually in Parts-Per-Million or ppm) proportionally increase the % of haemoglobin bound by carbon monoxide. Going from a concentration of 10ppm to 30ppm (all concentrations which are below the trigger level of domestic CO alarms) increases the percentage of carboxyhaemoglobin in the blood from 1.6% to 4.7%.

Generally, recommendations for maximum long-term CO exposure in the air vary from 20 to 30ppm. The UK’s Health and Safety Executive (HSE) states a maximum Workplace Exposure Limit during an 8 hr period of 20ppm carbon monoxide One could argue that for long-term domestic CO exposure in excess of 8 hours, an even lower concentration should apply.

Kidde 10LLDCO long life CO alarm with digital displayThe Kidde 10LLDCO can measure small amounts of CO

Yet, the British Standard for domestic Carbon Monoxide detectors BS EN 50291-1:2018 does not require a CO detector to trigger an alarm below 50ppm. As a result many CO alarms will not warn of potentially harmful long-term exposures between 20 and 50ppm. The gas used for the British standard conformity certification actually has a concentration of 50 to 60ppm; in the extreme the supplier can only assure buyers that the alarm will actually react at nearly 60ppm.

What can be done in this situation to detect long-term low level exposure?

Kidde offers a digital CO alarm, featuring an LCD display of the actual readings. While it, like all other kitemarked CO alarms, is also certified to alarm at over 50ppm, it already starts displaying readings from 11ppm, offering the user the information how much CO is actually present. Importantly, it also allows to display a peak reading by pressing a button. This peak reading will show you the highest concentration measured, e.g. in your absence or while you were asleep.


Carbon monoxide alarms are absolutely essential to protect you against dangerously high levels of the toxic gas. Wherever boilers, stoves and fire places are present, it is an essential to be protected by these alarms.

However, to really protect against low level carbon monoxide poisoning you require a detector with digital readout.


(doc:532 V1.0). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.


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