The world’s most widely cultivated edible fungi are: the common white button mushroom, Agaricus bisporus, with an annual production in 1991 of approximately 1.6 million tonnes; the oyster mushroom, Pleurotus spp. (about 1 million tonnes); and the shiitake, Lentinus edodes (about 0.6 million tonnes) (Chang 1993). Agaricus is mainly grown in the western hemisphere, whereas oyster mushrooms, shiitake and a number of other fungi of lesser production are mostly produced in East Asia.
The production of Agaricus and the preparation of its substrate, compost, are for a large part strongly mechanized. This is generally not the case for the other edible fungi, although exceptions exist.
The Common Mushroom
The common white button mushroom, Agaricus bisporus, is grown on compost consisting of a fermented mixture of horse manure, wheat straw, poultry manure and gypsum. The materials are wetted, mixed and set in large heaps when fermented outdoors, or brought into special fermentation rooms, called tunnels. Compost is usually made in quantities of up to several hundred tonnes per batch, and large, heavy equipment is used for mixing heaps and for filling and emptying the tunnels. Composting is a biological process that is guided by a temperature regime and that requires thorough mixing of the ingredients. Before being used as a substrate for growth, compost should be pasteurized by heat treatment and conditioned to get rid of the ammonia. During composting, a considerable amount of sulphur-containing organic volatiles evaporates, which can cause odour problems in the surroundings. When tunnels are used, the ammonia in the air can be cleaned by acid washing, and odour escape can be prevented by either biological or chemical oxidation of the air (Gerrits and Van Griensven 1990).
The ammonia-free compost is then spawned (i.e., inoculated with a pure culture of Agaricus growing on sterilized grain). Mycelial growth is carried out during a 2-week incubation at 25 °C in a special room or in a tunnel, after which the grown compost is placed in growing rooms in trays or in shelves (i.e., a scaffold system with 4 to 6 beds or tiers above each other with a distance of 25 to 40 cm in between), covered with a special casing consisting of peat and calcium carbonate. After a further incubation, mushroom production is induced by a temperature change combined with strong ventilation. Mushrooms appear in flushes with weekly intervals. They are either harvested mechanically or hand-picked. After 3 to 6 flushes, the growing room is cooked out (i.e., steam pasteurized), emptied, cleaned and disinfected, and the next growing cycle can be started.
Success in mushroom cultivation depends heavily on cleanliness and prevention of pests and diseases. Although management and farm hygiene are key factors in disease prevention, a number of disinfectants and a limited number of pesticides and fungicides are still used in the industry.
Electrical and mechanical equipment
A pre-eminent risk in mushroom farms is the accidental exposure to electricity. Often high voltage and amperage is used in humid environments. Ground fault circuit interrupters and other electrical precautions are necessary. National labour legislation usually sets rules for the protection of labourers; this should be strictly followed.
Also, mechanical equipment may pose dangerous threats by its damaging weight or function, or by the combination of both. Composting machines with their large moving parts require care and attention to prevent accidents. Equipment used in cultivation and harvesting often has rotating parts used as grabbers or harvesting knives; their use and transport require great care. Again, this holds for all machines that are moving, whether they be self-propelled or pulled over beds, shelves or rows of trays. All such equipment should be properly guarded. All personnel whose duties include handling electrical or mechanical equipment in mushroom farms should be carefully trained before work is started and safety rules should be adhered to. Maintenance ordinances of equipment and machines should be taken very seriously. A proper lockout/tagout programme is needed as well. Lack of maintenance causes mechanical equipment to become extremely dangerous. For example, breaking pull chains have caused several deaths in mushroom farms.
Physical factors such as climate, lighting, noise, muscle load and posture strongly influence the health of workers. The difference between ambient outside temperature and that of a growing room can be considerable, especially in the winter. One should allow the body to adapt to a new temperature with every change of location; not doing so may lead to diseases of the airways and eventually to a susceptibility to bacterial and viral infections. Further, exposure to excessive temperature changes may cause muscles and joints to become stiff and inflamed. This may lead to a stiff neck and back, a painful condition causing unfitness for work.
Insufficient lighting in mushroom-growing rooms not only causes dangerous working conditions but also slows down picking, and it prevents pickers from seeing the possible symptoms of disease in the crop. The lighting intensity should be at least 500 lux.
Muscle load and posture largely determine the weight of labour. Unnatural body positions are often required in manual cultivation and picking tasks due to the limited space in many growing rooms. Those positions may damage joints and cause static overload of the muscles; prolonged static loading of muscles, such as that which occurs during picking, can even cause inflammation of joints and muscles, eventually leading to partial or total loss of function. This can be prevented by regular breaks, physical exercises and ergonomic measures (i.e., adaptation of the actions to the dimensions and possibilities of the human body).
Chemical factors such as exposure to hazardous substances create possible health risks. The large-scale preparation of compost has a number of processes that can pose lethal risks. Gully pits in which recirculation water and drainage from compost is collected are usually devoid of oxygen, and the water contains high concentrations of hydrogen sulphide and ammonia. A change in acidity (pH) of the water may cause a lethal concentration of hydrogen sulphide to occur in the areas surrounding the pit. Piling wet poultry or horse manure in a closed hall may cause the hall to become an essentially lethal environment, due to the high concentrations of carbon dioxide, hydrogen sulphide and ammonia which are generated. Hydrogen sulphide has a powerful odour at low concentrations and is especially threatening, since at lethal concentrations this compound appears to be odourless because it inactivates human olfactory nerves. Indoor compost tunnels do not have sufficient oxygen to support human life. They are confined spaces, and testing of air for oxygen content and toxic gases, wearing of appropriate PPE, having an outside guard and proper training of involved personnel are essential.
Acid washers used for removal of ammonia from the air of compost tunnels require special care because of the large quantities of strong sulphuric or phosphoric acid that are present. Local exhaust ventilation should be provided.
Exposure to disinfectants, fungicides and pesticides can take place through the skin by exposure, through the lungs by breathing, and through the mouth by swallowing. Usually fungicides are applied by a high-volume technique such as by spray lorries, spray guns and drenching. Pesticides are applied with low-volume techniques such as misters, dynafogs, turbofogs and by fumigation. The small particles that are created remain in the air for hours. The right protective clothing and a respirator that has been certified as appropriate for the chemicals involved should be worn. Although the effects of acute poisoning are very dramatic, it should not be forgotten that the effects of chronic poisoning, although less dramatic at first glance, also always require occupational health surveillance.
Biological agents can cause infectious diseases as well as severe allergic reactions (Pepys 1967). No human infectious disease cases caused by the presence of human pathogens in compost have been reported. However, mushroom worker’s lung (MWL) is a severe respiratory disease that is associated with handling the compost for Agaricus (Bringhurst, Byrne and Gershon-Cohen 1959). MWL, which belongs to the group of diseases designated extrinsic allergic alveolitis (EAA), arise from exposure to spores of the thermophilic actinomycetes Excellospora flexuosa, Thermomonospora alba, T. curvata and T. fusca that have grown during the conditioning phase in compost. They can be present in high concentrations in the air during spawning of phase 2 compost (i.e., over 109 colony-forming units (CFU) per cubic metre of air) (Van den Bogart et al. 1993); for causation of EAA symptoms, 108 spores per cubic metre of air are sufficient (Rylander 1986). The symptoms of EAA and thus MWL are fever, difficult respiration, cough, malaise, increase in number of leukocytes and restrictive changes of lung function, starting only 3 to 6 hours after exposure (Sakula 1967; Stolz, Arger and Benson 1976). After a prolonged period of exposure, irreparable damage is done to the lung due to inflammation and reactive fibrosis. In one study in the Netherlands, 19 MWL patients were identified among a group of 1,122 workers (Van den Bogart 1990). Each patient demonstrated a positive response to inhalation provocation and possessed circulating antibodies against spore antigens of one or more of the actinomycetes mentioned above. No allergic reaction had been found with Agaricus spores (Stewart 1974), which may indicate low antigenicity of the mushroom itself or low exposure. MWL can easily be prevented by providing workers with powered air-purifying respirators equipped with a fine dust filter as part of their normal work gear during spawning of compost.
Some pickers have been found to suffer from damaged skin of finger tips, caused by exogenous glucanases and proteases of Agaricus. Wearing gloves during picking prevents this.
Mushroom growing has a short and complicated growing cycle. Thus managing a mushroom farm brings worries and tensions which may extend to the workforce. Stress and its management are discussed elsewhere in this Encyclopaedia.
The Oyster Mushroom
Oyster mushrooms, Pleurotus spp., can be grown on a number of different lignocellulose-containing substrates, even on cellulose itself. The substrate is wetted and usually pasteurized and conditioned. After spawning, mycelial growth takes place in trays, shelves, special containers or in plastic bags. Fructification takes place when the ambient carbon dioxide concentration is decreased by ventilation or by opening the container or bag.
Health risks associated with the cultivation of oyster mushrooms are comparable to those linked to Agaricus as described above, with one major exception. All Pleurotus species have naked lamellae (i.e., not covered by a veil), which results in the early shedding of a large number of spores. Sonnenberg, Van Loon and Van Griensven (1996) have counted spore production in Pleurotus spp. and found up to a billion spores produced per gram of tissue per day, depending on species and developmental stage. The so-called sporeless varieties of Pleurotus ostreatus produced about 100 million spores. Many reports have described the occurrence of EAA symptoms after exposure to Pleurotus spores (Hausen, Schulz and Noster 1974; Horner et al. 1988; Olson 1987). Cox, Folgering and Van Griensven (1988) have established the causal relation between exposure to Pleurotus spores and occurrence of EAA symptoms caused by inhalation. Because of the serious nature of the disease and the high sensitivity of humans, all workers should be protected with dust respirators. Spores in the growing room should at least partially be removed before workers enter the room. This can be done by directing the circulation air over a wet filter or by setting ventilation at full power 10 minutes before workers enter the room. Weighing and packing of mushrooms can be done under a hood, and during storage the trays should be covered by foil to prevent release of spores into the working environment.
In Asia this tasty mushroom, Lentinus edodes, has been grown on wood logs in the open air for centuries. The development of a low-cost cultivation technique on artificial substrate in growing rooms rendered its culture economically feasible in the western world. The artificial substrates usually consist of a wetted mixture of hardwood sawdust, wheat straw and high-concentration protein meal, which is pasteurized or sterilized before spawning. Mycelial growth takes place in bags, or in trays or shelves, depending on the system used. Fruiting is commonly induced by temperature shock or by immersion in ice-cold water, as is done to induce production on wood logs. Due to its high acidity (low pH), the substrate is susceptible to infection by green moulds such as Penicillium spp. and Trichoderma spp. Prevention of the growth of those heavy sporulators requires either sterilization of the substrate or use of fungicides.
The health risks associated with the cultivation of shiitake are comparable with those of Agaricus and Pleurotus. Many strains of shiitake sporulate easily, leading to concentrations of up to 40 million spores per cubic metre of air (Sastre et al. 1990).
Indoor cultivation of shiitake has regularly led to EAA symptoms in workers (Cox, Folgering and Van Griensven 1988, 1989; Nakazawa, Kanatani and Umegae 1981; Sastre et al. 1990) and inhalation of spores of shiitake is the cause of the disease (Cox, Folgering and Van Griensven 1989). Van Loon et al. (1992) have shown that in a group of 5 patients tested, all had circulating IgG-type antibodies against shiitake spore antigens. Despite the use of protective mouth masks, a group of 14 workers experienced a rise in antibody titres with increased duration of employment, indicating the need for better prevention, such as powered air-purifying respirators and appropriate engineering controls.
Acknowledgement: The view and results presented here are strongly influenced by the late Jef Van Haaren, M.D., a fine person and gifted occupational health physician, whose humane approach to the effects of human labour was best reflected in Van Haaren (1988), his chapter in my textbook that formed the basis of the present article.