Hydrogen Sulphide (H2S)

Often produced from breakdown of organic matter in the absence of oxygen, so can be a risk in tanks, sewers, treatment plants, farming, refuse and recycling plants. The density of H2S means it tends to sink and build up at low levels. While at lower concentrations it can smell like rotten eggs, prolonged exposure means that the nose can no longer detect it. Effects of inhalation and facial exposure range from eye irritation, sore throat and respiratory tract and shortness of breath at low concentrations, to burning eyes, corneal erosion, collapse, respiratory paralysis, convulsions, heart arrythmia and death at high concentrations. Hydrogen Sulphide is also a highly flammable gas and gas/air mixtures can be explosive. If ignited, further toxic vapours and gases may be produced.

Carbon Monoxide

Colourless, tasteless, odourless and non-irritating, CO is a dangerous gas which is usually produced as a by-product of the incomplete combustion of fuel. It can kill without warning – when inhaled, it attaches itself to the body’s oxygen carrier, haemoglobin, so dangerously reducing the oxygen being carried around the body. Just a brief exposure can cause flu-like symptoms - headaches, nausea, dizziness, muscle pain and personality changes. More will lead to movement problems, weakness, confusion, heart and lung problems, loss of consciousness and death.

HSE sets a short-term exposure limit of 10 ppm for any 10-15 minute period, or 5ppm over an 8 hour period. Low alarms should be set at or below 10ppm, high alarms at or below 20ppm.

HSE sets a maximum exposure limit of 20 ppm over an 8 hour period. Physical symptoms may set in at 35ppm, but exposure to low levels over a longer period of time can still leave the sufferer with symptoms easily confused with common illnesses.

Oxygen (O2)

Leaks in pressured systems or pressure relief valves, excess oxygen used in cutting or welding processes and spills of liquid oxygen can all lead to an excess of oxygen in the environment, amongst other industrial processes. Oxygen’s ability to accelerate burning enables materials to ignite much more easily, burning much faster and hotter in an oxygen enriched atmosphere, as can be seen with the effects of a ‘backdraft’ effect when opening a door onto an established fire. High oxygen concentrations in inhaled air can cause inflammation and lung damage, neurological problems, vision loss, cell and tissue damage, loss of consciousness and death.

Sulphur Dioxide (SO2)

With purposes as wide as fumigation, food preservation and bleaching, the chemical SO2 is frequently used in industry, as well as being released from the burning of fossil fuels in power plants, refineries, motors and boilers. It is colourless, but with a pungent smell. Inhalation causes irritation and at higher levels, corrosion, to the respiratory tract, and similar effects to the gastrointestinal tract. Eye contact causes irritation and, in severe cases, blindness.

Despite being essential for life, oxygen levels for regular breathing air should always be between 19.5% and 23.5%. Any lower and the air is deemed oxygen deficient, but any higher and there is the risk of both oxygen toxicity and severe fires and explosions.

HSE sets an exposure limit of 0.5 ppm over an 8-hour period but also specifies that high levels can be life threatening even with short term exposure.