inert and noxious asphyxiants

inert asphyxiants - carbon dioxide

Co2 replaces oxygen but does not have a directly toxic effect on biological tissue. It is a colourless, odourless gas that is denser than air.

It causes rapid breathing, fainting and unconsciousness.

The greatest risks of carbon dioxide poisoning are after volcanic eruptions. There are few warning signs.

lake Nyos, Cameroon

The lake overturned in 1986, releasing dissolved volcanic CO2.

1,700 people, plus numerous animals, asphyxiated up to 25 km from the lake, but survivors show no long-term health effects.

lake Kivu volcanic activity

Lake Kivu and Goma city lie on the East African rift including Mount Nyiragongo. Volcanic gases are greatest hazard, from lava flows at mazukus and saturating bottom water of Lake Kivu.

Ongoing asphyxiation hazard at mazukus, such as these beside refugee camp near Goma city, where carbon dioxide seeps from lava flows.

Lake Kivu is stratified, with lower layers containing much dissolved CO2 and methane. Risk of overturn of lake Kivu, leading to Lake Nyos-type event, but with methane explosion. Methane is now being extracted from these deep waters and burned for power production; CO2 not expected to reach overturn pressure in foreseeable future.

CO2 suppresses O2 binding to haemoglobin 

CO2 is an allosteric inhibitor of haemoglobin: its binding decreases the protein's ability to bind with oxygen. CO2 reacts with water to form carbons and (H2CO3) which dissociates into bicarbonate (H2CO3) and H+ ions. The decreased pH reduces the ability of haemoglobin for oxygen.

In the figure, a higher partial pressure is needed to saturate haemoglobin to a given level if there is more CO2 in the air.

hydrogen sulphide

• Colourless, denser than air. flammable

• Rotten eggs smell, but odourless at >~100ppm (paralysis of olfactory nerve) and lethal above ~300 ppm.

carbon monoxide

• Colourless and odourless; denser than cold air, lighter than warm air

• Binds 200 times more strongly than oxygen to haemoglobin; small concentrations are deadly within hours (~1000 ppm) or minutes (~10.000 ppm)

symptoms of hydrogen sulphide poisoning

Effects with increasing concentration include:

1. Irritation to eyes and throat

2. Pulmonary oedema

3. Noxious asphyxia by displacing oxygen from haemoglobin Long-term effects to low level exposure not well known

volcanic H2S poisoning appears to be rare but:

Yellowstone, USA

Possible H2S pooling in depressions, like carbon dioxide; reports of wild animal deaths

Fukushima, Japan

3 men died of suspected H2S poisoning near an onsen (volcanic warm bath) in February

2025

symptoms of carbon monoxide poisoning

Once carbon monoxide binds to a site, the haemoglobin holds oxygen more strongly, preventing its release to body tissues.

Long-term effects: neurological problems in adults, learning disabilities and developmental issues in children and miscarriage in women during pregnancy.

Remedy: supply 100% oxygen, to forcibly substitute oxygen for CO molecules