Hazardous Gases
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IntroductionHazardous gases on farms are found in silos, manure storages, grain bins, and barns. These structures provide a confined space in which certain gases may accumulate to dangerous levels. When plant material is stored in a silo, fermentation takes place, a process that allows the crop to be stored for a long period of time. However, the fermentation process uses up oxygen and produces carbon dioxide and nitrogen dioxide as by-products. This can lead to an environment which is unsuitable for humans soon after the silo is filled. When manure is stored for a long period of time, it undergoes anaerobic decomposition. During this process, manure gases are produced. Warm weather and poor ventilation can increase the concentration of these gases. Liquid manure tanks therefore can produce toxic levels of gases, or result in a lack of oxygen. Agitating manure in a liquid storage also results in a rapid increase in the release of manure gases. In addition to the human risks, high gas levels can deteriorate exposed concrete above the liquid manure surface. Explosions have taken place when methane gas was allowed to accumulate in an enclosed space. Clause 78, Part VIII in Ontario Regulation 267/03 created under the Nutrient Management Act, 2002, requires all new liquid manure storages provide some form of ventilation to prevent the accumulation and or intensification of corrosive, noxious or explosive gases. Silo and manure storages contain a variety of gases. These gases may be classified as irritants or asphyxiants. Irritants cause inflammation and irritation to the respiratory system tissues. Asphyxiants are gases that displace oxygen from the air (simple asphyxiants), or combine with the blood's hemoglobin (chemical asphyxiants). This Factsheet focuses on dangerous gases which can be found around farms and safety precautions which will help protect farm workers from these silent killers. For information on health problems related to respirable dust, refer to OMAFRA Factsheet, Farm Workers Health Problems Related to Air Quality inside Livestock Barns, Order No. 93-003. Hydrogen SulphideHydrogen sulphide, H2S, is the most dangerous manure gas. It is classified as a chemical asphyxiant because it immediately chemically interacts with the blood's hemoglobin to block oxygen from being carried to the body's vital organs and tissues. It is produced from the anaerobic decomposition of organic materials such as manure. Its characteristic rotten egg smell is easy to detect at low concentrations, but at higher concentrations it paralyses the sense of smell. This can give someone a false sense of security when exposed to hydrogen sulphide. In high concentrations, hydrogen sulphide causes instant paralysis and death. Table 1. Hydrogen Sulphide Effects on Humans at Various Concentrations
Source: American Society of Agricultural Engineering Standards, 2003) Table 1 outlines the effect of hydrogen sulphide at various concentrations. Hydrogen sulphide is heavier than air; therefore, it tends to be located just above the surface of the manure. The release of hydrogen sulphide is relatively low when manure remains undisturbed and the outside temperature is low. However, hydrogen sulphide levels can reach dangerous levels very quickly when a tank is agitated, especially if splashing or surface agitation takes place (see Figure 1). Extra precautions are required when dealing with an under floor manure storage (see Management Suggestions - Manure Storages). Several deaths have occurred when workers have entered a manure storage or the room above one in an attempt to save someone who had been overcome by H2S. Figure 1. H2S concentration during pit agitation. (Patni, N. K. and S. P. Clarke, Ontario Swine Research Review, 1990) MethaneMethane, CH4, is a colourless and odourless gas. This combustible gas is generated by anaerobic digestion of organic material and if stored can be used as a fuel source for internal combustion engines. It is lighter than air, therefore, tends to rise from the manure storage. Methane is non-toxic and is unlikely to be a concern in well-ventilated livestock buildings. In covered and in-barn storages, methane can become trapped and the concentration can reach dangerously explosive levels. In 1996, an Ontario farmer sat down at his office desk after doing the morning feeding and proceeded to light a cigarette. The office, washroom, feed room, and connecting hallway were constructed on top of a concrete covered manure storage that was located between two parallel swine barns. The tank was almost filled to capacity and due to be pumped out. The shower facility had not been actively used for several years and likely did not have any water in the drain trap, causing methane to escape into the office for some time. The spark ignited the methane gas that had accumulated in the office area causing an explosion and small fire. Although the office was destroyed, fortunately the farmer managed to escape with second degree burns to his hands. AmmoniaAmmonia, NH3, is a colourless gas and has a characteristically pungent odour. It is produced by the decomposition of animal manures. This gas is classified as an irritant. It is lighter than air and can pre-dispose livestock to various respiratory diseases if exposed to a significant gas level for an extended period of time. Ammonia irritates the eyes at levels in the range of 20-50 PPM depending on the sensitivity of the person or animal. This gas tends to be a concern mainly in swine and poultry buildings however, it can also be a problem in manure composting operations. As a guideline, if eye irritation occurs, the ventilation in the building should be improved. Carbon DioxideCarbon dioxide, CO2, is colourless and odourless. It is, in part, the product of respiration of both plant material and animals, and is found naturally in the atmosphere. All open flame, non-vented, space heaters will also contribute carbon dioxide to the surrounding air space as one of the products of combustion. It is heavier than air and, like hydrogen sulphide, will tend to accumulate just above the animal pen floor, surface of manure in a manure tank, or silage surface in a silo. The main danger with carbon dioxide is that it can create an oxygen deficiency and can result in asphyxiation or suffocation. Well-ventilated livestock buildings do not generally contain dangerous levels of CO2, however, lethal concentrations can be found in sealed silos, liquid manure storages, and grain storages. As part of the ensiling process, living plant material quickly uses up available oxygen and dies. During this respiration process, oxygen is converted to water and CO2. Carbon dioxide displaces the oxygen in a sealed silo, making this environment unsuitable for humans without an external air supply. Nitrogen Dioxide (NO2)Nitrogen dioxide is a dangerous chemical asphyxiant and is produced as a result of chemical reactions that take place almost immediately after plant material is placed into a silo. Even short-term exposure can result in sudden death. It has a characteristic bleach-like odour and may be visible as a reddish-brown haze. It is heavier than air; therefore, it will tend to be located just above the silage surface. It may also flow down silo chutes and into feed rooms. Weather conditions and cultural practices will affect the amount of nitrates in plant material, which in turn will set the stage for the production of NO2 in the silo. For example, a dry period during the growing season followed by abundant rainfall will encourage a corn crop to take up high levels of dissolved nitrates. If the corn is harvested before the nitrates can be converted to proteins, nitrous oxide (N2O) and nitric oxide (NO) are produced. Unstable NO combines with oxygen to form deadly nitrogen dioxide. When inhaled, NO2 dissolves in the moisture on the internal lung surface to produce a strong acid called nitric acid. Nitric acid burns the lung tissues, which is followed by massive bleeding and death. Repeated exposure to lower concentrations of NO2 will cause chronic respiratory problems, including shortness of breath, coughing and fluid in the lungs. Safe Gas LevelsThe American Conference of Government Industrial Hygienists has established maximum safe gas concentrations, or threshold limit values, for an 8 hour work day and 40 hour work week for humans (see Table 2). Although threshold limit values have not been established for animals, many researchers have suggested that animal responses are likely similar to humans. Table 2. Threshold Limit Values (time weighted average) for maximum gas concentrations in humans
(Source: American Conference of Government Industrial Hygienists) Gas Detection MonitorsAll of these hazardous gases can be measured with appropriate test equipment. Although much of the equipment is expensive and requires periodic re-calibration, there are some very reasonable gas detection tubes and hand held monitors commercially available from safety and scientific supply stores. A box of 10 gas detector tubes is less than $100 and hand held monitors start at about $250. All of these gases can be measured with reactor tubes in which the gas changes the colour of a reactant and gives a good estimate of its concentration. These reactor tubes can be used with a calibrated pump to draw a measured sample of gas through the tube and obtain the result within a few minutes. These tubes are also available as passive dositubes which react slowly when exposed to the environment containing the gas of interest and yield an average gas concentration over time. Depending on the gas, a passive dositube would be exposed to the environment being checked for several hours to as much as 48 hours. Hand held monitors (see Figure 2) are now available for not only measuring these gases, but they are also equipped to sound an alarm when a dangerous gas level is detected. In fact, safety consultants are recommending the monitoring units with alarm only and no digital readout so that the person is not inclined to take the extra time to check the actual gas level before making a hasty exit. Figure 2. Example of commercially available monitor for hydrogen sulphide gas. (Source: Agviro Inc.) Management SuggestionsManure Storages
Liquid Manure Tankers
Figure 3. Safety hatch for liquid manure tanker. Silos
Figure 4. Suggested ventilation adapter for rotary distributors.
Figure 5. Suggested ventilation adapter for fin-type distributors. ConclusionNever assume that the environment inside a silo or manure storage is adequate. Do not enter a liquid manure tank or recently filled silo, under any circumstances, without a pressure-demand remote breathing apparatus. These confined spaces often contain lethal concentrations of hazardous gases. Always have a lifeline attached, with a responsible safety person in view of your work. Follow the management suggestions outlined in this factsheet and post clearly visible warning signs to warn others to stay away. Additional Information
For more information: Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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