Following the death of a teenager in 2005 due to hydrogen sulphide gas poisoning during the removal of stored spent mushroom compost for land-spreading, Teagasc has now characterised emissions of the toxic gas associated with this activity and developed guidelines to ensure the safety of operators.
In the agricultural world, fatal accidents occur every year that are due to poisoning by toxic hydrogen sulphide gas (H2S), also known as ‘manure’ gas or ‘slurry’ gas. The reality is that this toxic gas can build up wherever organic-rich material is stored for periods of time, such as liquid manure tanks, slurry tanks, and heaps of organic compost. Under these conditions, the material quickly becomes anaerobic, which favours the growth of bacteria that produce toxic H2S gas as a by-product. The gas builds up within the liquid slurry or the compost heap and it is only released into the atmosphere when the slurry or compost is disturbed. A plume of concentrated gas is then released which can kill people and animals within minutes. Concentrations above 500ppm can cause unconsciousness and death in less than a minute while concentrations of 20-500ppm can cause eye and lung irritation and damage. Statutory exposure limits exist to protect workers and, in many countries, including Ireland, these are currently set at 10ppm for 15 minutes (short-term exposure limit– STEL) and 5ppm for eight hours (time weighted average – TWA). It has been recommended to reduce these to 5ppm (STEL) and 1ppm (TWA). The dangers of slurry-tank gas are generally widely known among the farming community, where deaths associated with slurry-tank mixing and emptying occur all too regularly. However, the dangers of working with liquid animal manures in other sectors, such as mushroom compost production, are less widely appreciated. A few deaths and serious accidents have occurred around the world associated with liquid manure storage and use at mushroom compost facilities, and these risks should be identified and addressed by compost companies in line with the relevant health and safety guidelines in individual countries. However, the first death due to H2S poisoning associated with disturbance and removal of stored spent mushroom compost was reported in Ireland in 2005 – identifying a heretofore unknown risk associated with this activity. Teagasc embarked on a programme of research to identify the patterns of H2S emissions in human - occupied zones during disturbance of spent mushroom compost and the results and recommendations are summarised below.
Outside and inside tractor cabs
Hydrogen sulphide (H2S) gas levels were monitored in the human-occupied zone at four Spent Mushroom Compost (SMC) storage sites during SMC removal for spreading on agricultural land. During SMC removal operations, H2S gas monitors were mounted on the outside of the tractor, positioned at the SMC periphery and worn by individual tractor drivers. The highest H2S concentrations at 10-second intervals were detected just outside the tractor cab as it was taking a load of SMC to put into a land- spreading trailer (Fig 1). Concentrations as high as 454ppm were detected at the outside storage sites and 214ppm at storage sites under cover. The tractor driver is within the closed cab of the tractor during this operation, but, none-the-less, concentrations inside the tractor cab reached as high as 100+ppm at the outdoor storage sites and 51ppm at the indoor storage sites (100ppm is the limit of detection of the personal monitors used by tractor drivers). Thus, H2S is clearly infiltrating the tractor cab during the SMC removal operations and, although the concentrations inside cabs are reduced to about a quarter of the concentration outside the cab, in many cases there is only one driver who operates the tractors of both vehicles – the SMC loader and the SMC spreader, so he is exposed to the higher concentrations outside the loader tractor cab when he dismounts. As H2S is a gas, the concentration in the air fluctuates as it is dispersed and diluted in the air by air currents and tractor movements, but as it is heavier than air, on still days and when there are no tractor movements there will be a tendency for it to settle in the vicinity and dissipate slowly. Concentrations as high as 250ppm were detected at the edges of the SMC storage areas, up to 10m from the tractor activity – indicating that toxic gas levels can still occur at a distance from the activity itself.
Outdoor vs. indoor SMC storage
H2S gas is produced from organic matter under anaerobic conditions associated with high moisture contents. Our research has indicated that SMCs stored under cover and protected from rain had average moisture contents of 53-65% compared with SMC stored outdoors where average moisture contents were 66 - 72%; and this was reflected in lower H2S emissions during compost disturbance from SMC stored under cover. The lowest H2S concentrations measured in this study were at a small SMC storage site where the average moisture content of the SMC was 53-63%. A larger heap stored undercover was not as dry at 62-65% moisture content and this was reflected in higher H2S emissions compared with the smaller site. Thus, where possible, SMC should be stored in smaller heaps that are protected from rainfall in order to facilitate drying- out and thereby reduce the potential for high levels of H2S to be produced.
The H2S levels at the large indoor stored heap were much higher than anticipated, given the much lower moisture content of the material. The large heap stored under cover also tended to get much hotter (36-51oC) than the smaller indoor heap (33-41oC) or outdoor stored heaps (24-36oC) and it is likely that the higher temperatures increase the activity of thermophilic H2S-producing bacteria, counteracting the reduced activity due to the lower moisture content, but that hypothesis needs to be tested.
Given this new information, those who store and handle SMC should review their management system to eliminate or reduce the risk to operators of H2S exposure in the workplace. A number of recommendations are given that aim (a) to reduce the potential for H2S emissions and (b) to change work practices to minimise exposure risks.
Recommendations to reduce the potential for H2S emissions from stored SMC:
1. Where possible, SMC should be stored under cover (i.e. open-sided barn construction) to prevent rainfall landing on the SMC, thereby decreasing the moisture content and reducing the potential for excessive H2S accumulation.
2. SMC should be stored in small heaps, where possible, to prevent excessive production and build-up of H2S associated with larger heaps.
3. SMC disturbance and removal should only be done when wind speeds are at least > 6m/s (moderate breeze) to facilitate dissipation and dilution of any H2S emissions.
Recommendations to minimise H2S exposure risks when working with stored SMC:
4. Tractor cabs should be maintained in a fit state of repair.
5. Drivers of SMC loader tractors should wear a personal H2S monitor and be trained in its operation.
6. Drivers of SMC loader tractors should take a short break every hour away from the SMC storage site
7. Drivers of SMC loader tractors should carry a full face gas mask fitted with an appropriate H2S filter in the tractor cab when working with SMC, in case of emergencies, and be trained in its use and maintenance
This article is based on a paper recently published in the Journal of Agricultural Safety and Health, where it received a 2014 ASABE Superior Paper Award from the
American Society of Agricultural and Biological Engineers. It is available on: http://t-stor.teagasc.ie/
Article Authors: Teagasc:Dr Helen Grogan, Dr Tom Curran, Balasubramanian Velusami