Bioaerosols and OSH
Rafał L. Górny, Central Institute for Labour Protection - National Research Institute, Poland
- 1 Introduction
- 2 Definitions
- 3 Biological and physical features of bioaerosol particles
- 4 Bioaerosol sources at workplaces
- 5 Health effects of bioaerosols
- 6 Exposure assessment and risk management
- 7 Threshold limit values for occupational bioaerosols
- 8 Legislative aspects
- 9 Prevention and control measures
- 10 New trends in bioaerosol measurement and control
- 11 References
- 12 Links for further reading
The presence of biological agents in occupational environments is an important medical and social problem. Micro- and macroorganisms and the structures and substances they produce may exert a harmful influence upon exposed individuals leading to numerous adverse health outcomes. Biological agents that are transmitted as bioaerosols are of the greatest epidemiological importance. On a global scale, at least several hundred million people are estimated to be exposed to these risks at work. From a public health perspective, the costs of hazardous exposure to bioaerosols are significant reaching billions of dollars.
|Actinomycetes||varied group of rod-shaped to filamentous Gram-positive bacteria|
|Aerodynamic diameter||diameter of a unit-density sphere having the same gravitational settling velocity as the particle being measured|
|Bioaerosol||airborne particles of biological origin|
|Biological agents||microorganisms, including those which have been genetically modified, cell cultures and human endoparasites, which may be able to provoke any infection, allergy or toxicity|
|cfu, colony forming units||number of microbial cells, spores or their aggregates which grow on agar media as separate colonies|
|Endotoxin||constituent of the external membrane of Gram-negative bacteria (lipopolysaccharide)|
|Glucans||water-insoluble structural cell-wall components of most of the fungi and yeasts, some bacteria and plants|
|Gram-positive/Gram-negative||terms describing specific staining characteristics of bacteria. Compared to Gram-positive bacteria (which are stained purple), Gram-negative bacteria typically have an additional outer membrane in their cell walls (and are stained pink)|
|Half-life||time interval required to reduce the rate of emission of particles in question (e.g. microbial particles) by a factor of two|
|Interception||collision with and deposition of a particle on an object when the particle passes within the distance of one particle radius of the object|
|Mycotoxins||toxic secondary metabolites produced by fungi|
|Propagule||reproductive particle released by an organism, e.g. a fungal spore|
Biological and physical features of bioaerosol particles
Sources and occurrence
Like particles of variable biological origin, bioaerosols are ubiquitous in both indoor and outdoor environments. Their major outdoor sources are soil, natural and anthropogenic water reservoirs as well as living and dead plants. Indoor sources can have both occupational and non-occupational characters. Occupational sources can be very productive and can create bioaerosol concentrations up to 1012 cfu/m3 (see chapter 3). Among non-occupational ones, the most important source is usually the presence of humans and their physical activities (e.g. breathing, talking, sneezing, coughing, scratching etc.). Bioaerosol particles are usually naturally present in the environment. The frequency of their appearance and occurrence depends on: climatic zones, continents, geographic regions, character of the biotope as well as occurrence and distribution of specific groups of plants or animals.
All these factors contribute to spatial variations of bioaerosol concentrations. Moreover, the environmental and temporal variations have been found to produce both diurnal and annual cyclic changes in the quantity and quality of bioaerosol particles. Time-dependent variations can range over several orders of magnitude as bioaerosol sources do not generate particles continuously. Microclimate parameters, i.e. temperature and relative humidity of the air, together with other physical parameters of the environment (e.g. ultraviolet radiation, oxygen forms and availability) contribute to the influx of airborne microorganisms. These factors also influence the survival of airborne microbes and affect their ability to colonise on surfaces after deposition.
Bioaerosol particles may be viable (i.e. able to reproduce or have metabolic activity) or non-viable (i.e. dead or unable to reproduce as whole cells or as their fragments). From human exposure point of view, both states are important. To be infectious, a biological agent must be viable, whereas allergenic and toxic properties can be preserved even after death of the agent. In the environment, viable bioaerosol particles, especially microorganisms, are not stable and undergo mutations (genotype changes), phenotype changes, evolution and relative selection (loss of current traits or gain of new traits by the succeeding generation). All these changes (when certain new attributes are acquired and/or other irretrievably lost) can lead to appearance of bioaerosol particles with “new” features, e.g. bacteria resistant to certain antibiotics or disinfectants, fungi resistant to fungicides, protozoans resistant to drugs.
Biological agents frequently pose a threat to individuals in form of bioaerosols. They mostly occur in the environment as particles. This feature determines their behaviour and dynamics in the air as well as decides about the place of their deposition within the specific spaces or on surfaces. Bioaerosols can penetrate into the human body through the nose, mouth and conjunctiva epithelium, bronchi and alveoli in the lung, as well as epidermis. In the human respiratory tract, the penetration depth and behaviour of bioaerosol particles depends on their sizes, shapes, densities, electrical charges, chemical composition and reactivity. Besides, the physiological factors such as airflow and breathing patterns influence the mechanism of particle deposition as well. A basic particle property is its size characterised by the particle diameter. Bioaerosol particles cover a wide size range from about 0.02 μm up to hundreds of micrometres. Only a small fraction of particles are spherical. Therefore particles of irregular shapes are characterised by means of a particle equivalent diameter.
To describe bioaerosol particles in exposure or health studies, the aerodynamic diameter is usually used. The aerodynamic particle sizes may significantly differ from their physical sizes. For example, the physical size of Pseudomonas fluorescens bacterial rods is between 0.7-0.8×1.5-3 μm and the corresponding aerodynamic diameter is 0.8 μm; for spherical spores of the fungus Aspergillus versicolor, the aerodynamic diameter of 0.8 μm is equivalent to a physical dimension of 2-3.5 μm.
Behaviour in the respiratory tract
Due to a high settling velocity, the particles with diameters bigger than 11 μm practically do not penetrate into the deeper respiratory tract. A vast majority of bacterial and fungal airborne particles are below this size being present as separate cells, spores, their fragments or as aggregates of the solely biological or biological and dust particles. If a particle is highly hygroscopic, its aerodynamic diameter may be significantly larger due to humidity of the air and moisture conditions e.g. in the lung.
- Most particles greater than 10 μm, and up to 80% of particles between 5-10 μm, are trapped in the nasopharyngeal region due to inertial impaction and centrifugal condensation resulting from the anatomic formation of these parts of the respiratory tract where the air stream has the highest velocity. For particles with an elevated ratio between their length and diameter e.g. for fungal spore chains, these processes are assisted by the interception mechanism. Particles trapped in upper parts of the respiratory tract are usually removed within a few hours by the mucociliary system or as a result of expectoration.
- Particles with diameters above 0.5 μm are deposited by sedimentation and impaction, which take place in bronchi, bronchioles, and alveoli, where the air velocity is low and the probability of deposition is directly proportional to the residence time. Particles deposited in the lower respiratory tract (bronchioles, alveolar ducts and alveoli, without ciliated epithelium) are removed much slower from several dozen to several hundred days.
- Particles of less than 0.5 μm are mainly separated from the air stream and deposited almost solely by diffusion. This process depends inversely proportional on the particle diameter and is supported by electrostatic precipitation resulting from interaction between surface and particle charges.
The type of interactions between the bioaerosol particles and human cells depends on the place of their deposition and is conditioned by their retention time in the respiratory tract:
- Inhaled particles with aerodynamic diameters equal or above 10 μm (e.g. pollens, fungal spores) cause eye or nose irritations.
- Particles, whose sizes are between 5 and 10 μm (e.g. fungal spores, bioaerosol aggregates) may provoke asthmatic symptoms. Those deposited in bronchi require up to 24 hours to be expelled.
- Particles below 5 μm (e.g. single fungal spores, bacterial cells, their fragments and aggregates with dust particles) may evoke allergic alveolitis type reactions.
- Field studies show that microbial particles with diameters below 2.5 μm (i.e. actinomycetal spores, most indoor fungal spores) are the most dangerous for human health if they are inhaled. Having abilities to avoid numerous defence systems in the respiratory tract (e.g. ciliated epithelium, mucus, saliva, etc.), they can burden the body with high concentrations of biologically active molecules. All of them are subsequently moved to the oral cavity and then directly or indirectly (i.e. through the alimentary tract) eliminated from the organism.
Stability and viability in the air
The time interval in which particles are airborne must be long enough to give them opportunity to be inhaled. In real situations when the air undergoes permanent turbulences, this factor can be characterised by the so-called ’half-life’. Briefly: despite the fact that bioaerosol particles are rarely spherical, their sedimentation velocity can be relatively precisely calculated. According to the Stokes’ law, it is dependent (among others) on the square of particle diameter. Based on that, for large bioaerosol particles of about 100 μm, estimated half-life equals to a few seconds, whereas for particles with aerodynamic diameters of about 10 μm, 3-1 μm and 0.5 μm half-lives increase roughly to a few minutes, hours and days, respectively.
The air as a biotope does not support the survival of biological agents. Nevertheless, several studies show that numerous fine particles of biological origin are able to maintain their viability (simultaneously preserving their immunological reactivity) in the air much longer than bigger organisms. For example: survival time for the bacterium Legionella pneumophila is less than 15 minutes, for Escherichia coli and Streptococcus faecalis between 30-60 minutes, for other streptococci up to 48 hours, for staphylococci about 3 days, whereas for influenza virus it may be extended up to 3 weeks, for spores of the fungi Aspergillus and Penicillium up to 12 years, mite allergens are decomposed within a few months. From the inhalation exposure point of view, an interrelation between stability and viability of bioaerosol particles determines the possible health outcomes.
Bioaerosol sources at workplaces
In many occupational environments workers can be exposed to a wide spectrum of bioaerosols. The extent to which they can be endangered depends on the source strength (see chapter 9) of biological contamination and the character of work activities being performed. The latter aspect is the resultant of both the abilities to generate and subsequently disperse biological particles into the air as well as the appropriateness of the control measures put in place to protect workers from the bioaerosols.
Even if the specific activity does not involve a deliberate intention to work with or use a biological agent, specific circumstances supporting unintended emission or multiplication of biological agents may result in workers' exposure above a certain level recognised as ‘normal’ for such environment. Therefore incidental generation of biological agents has to be taken into account in the risk assessment.
Several examples of occupational activities where the exposure to bioaerosols is the most frequent and pronounced in the occupational environments are presented in Table 1. Dutkiewicz et al. (2007) identified exposure to biological agents for 151 occupational groups in 22 main branches of industry.
Health effects of bioaerosols
Bioaerosols of working environments may be:
- agents that cause infective and invasive diseases (e.g. viruses, bacteria, fungi);
- allergens (of bacterial, fungal, plant and animal origin);
- toxins and biological compounds with similar toxic effects (e.g. bacterial endotoxins, glucans);
- carcinogens (e.g. fungal mycotoxins such as aflatoxins, ochratoxins or trichothecenes);
- immunologically reactive fragments (submicro- and nano-metric particles of bacterial or fungal origin).
Until quite lately occupational biohazards were commonly assimilated with infectious agents. Such point of view is still reflected in the current EU legislation. In the majority of recently published scientific papers, an integrative concept which takes into account an airborne transport of harmful biological agents as the most common route for their dispersion has been accepted.
Bioaerosols can be responsible for many adverse health effects from allergic reactions, through infections, to toxic reactions and other non-specific symptoms referred to e.g. as ‘sick building syndrome’ (SBS) or ‘mucous membrane syndrome’ (MMS, commonly results in a dry cough, irritation of the eyes, nose and throat and may represent an irritant effect not involving immune responses or mediators). In most situations the observed outcomes are a result of mixed exposure to toxins and allergens.
Exposure assessment and risk management
The report ‘Expert forecast on Emerging Biological Risks related to Occupational Safety and Health’ concluded that difficulties to conduct assessment of biological risks, lack of proper information, inadequate training or poor knowledge, together with poor maintenance of working environment and inadequate emergency response plan to biological risks result in both poor risk management and poor prevention practices. In contrast to chemical or physical hazards, an unawareness of the biological contamination problem existence among both the local authorities and employers ends in a relinquishment of their control posing a real threat to workers.
A simple algorithm of action in exposure assessment (Figure 1) should always be applied to eliminate the above mentioned shortcomings.
A clear description of the dose-response relationship for the majority of biological agents cannot be established. One of the reasons seems to be the inadequacy of analytical methods applied in bioaerosol exposure assessment. Spatial and temporal variations usually do not permit measurement of the maximum possible concentration of the microbial agent.
- Traditional methods of bioaerosol sampling and analysis focus on evaluation of viable microbial spores and vegetative cells, omitting the role of non-viable propagules and small fragments of their structural elements. To be infectious bioaerosol particles must be viable; however, non-viable aerosol particles of biological origin can still be immunologically reactive and cause numerous adverse outcomes like allergies or toxic responses. This fact should be taken into account when exposure assessment is performed as the most commonly applied measurement procedures, based on bioaerosol viability at the level of 1-25%, significantly underestimate the real exposure. The most popular bioaerosol measurement methods underestimate the real exposure as they are measure a small part of bioaerosol particles only.
- The duration of sampling is usually short and, due to environmental, spatial and temporal variations, does not adequately represent the real degree of environmental contamination.
- Moreover, results can be biased by the time of measurement, i.e. microbial propagule release into the air may be sporadic, irregular, dependent on physical factors, sensitive to the specific environmental conditions (e.g. bioaerosol may not be well mixed in the air) and may not correspond well with the sampling time.
All of these may result in collection of unrepresentative samples. Hence, the exposure evaluation should contain, as its immanent part, the source identification and, if it is possible, reliable measure of microorganisms.
Source identification is often carried out by surface sampling using different techniques (e.g. transparent sticky tape, swab sampling or contact plates). All these methods can identify the source but cannot evaluate the magnitude of emission of microbial propagules into the surrounding air. Hence, the surface sampling is crude, expected to yield a poor surrogate of airborne concentrations and its results should be interpreted with caution.
Several studies have shown that personal exposure to particles is usually higher than the concentration measured simultaneously with stationary samplers. In epidemiological studies which relied on personal sampling of microorganisms, exposure-response associations were found almost twice as often as in studies using stationary sampling. As stationary sampling appears to underestimate the bioaerosol exposure, personal sampling is thus the method of choice.
At present, there are no worldwide distinct regulations to standardise bioaerosol sampling in occupational environments. Hence, the guidelines contained in four European standards (Table 2) published by CEN TC 137 describing the methods of workplace control can be applied for this purpose. They simultaneously provide a base for development of more detailed guidelines and/or instructions for bioaerosol assessment and control in individual countries as it was done for example by the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA).
Threshold limit values for occupational bioaerosols
On the global scale, there is a lack of acceptable occupational exposure limits (or threshold limit values) for bioaerosols. The main reason for this is the lack of well-documented and epidemiologically proven dose-response relationships between the exposure to specific biological agents and adverse health effects caused by their exact dose(s). Moreover, the sensitivity to each organism is individual and the strength of immunological reaction to the specific agent(s) is usually not the same in everybody. Despite progress in development of aerosol sampling techniques and analytical methods over the last two decades, the worldwide scientific database on bioaerosols is still insufficient to quantitatively and qualitatively characterise them. If threshold limit or reference values are established, they are usually connected with the clinical picture of the specific disease caused by the agent, taking into consideration its presence in a certain element of the environment only. Nevertheless, despite these limitations, the reference values expressed in numbers are applied to facilitate interpretation of the measurement data.
An appropriate elaboration strategy of threshold limit values for bioaerosols takes into consideration the research method and several environmental, source, quantitative and qualitative criteria, which play a key role in such a process. Figure 2 illustrates their interdependencies.
From the point of view of clinical outcomes caused by exposure to bioaerosols, the ideal situation would be as follows:
- A hygienic standard is created based on the relationship between exposure, type and concentration of biological agents and their health effects.
- The relationship would be epidemiologically proven.
- Experimental evidence proving this relationship would become known.
- The relationship would be clinically proven, i.e. medical evidence that a specific agent was responsible for certain specific health effect exists.
Today, this ideal situation does not exist for any biological agent. Therefore, due to the lack of possibilities to determine the relationship between the dose of biological agent(s) and health outcome in a precise way, the creation of a threshold limit value based on the "environmental philosophy" seems to be a reasonable alternative for the “clinical” approach.
Reference values derived from multiple biological agent concentration measurements, should enable an evaluation of the quality of environment, as well as the determination of “what is typical or acceptable” and “what is atypical or inacceptable” for a specific type of environment (or its specific area). The threshold limits hitherto available in the scientific literature for bioaerosols in occupational environments are as follows:
- for total number of bacteria: ≤1.0×103-7.0×103 cfu/m3 for non-industrial workplaces and ≤7.5×102-1.0×107 cfu/m3 for manufacturing and industrial premises; for pathogenic microorganisms, there is no safety level (the threshold limit should be 0 cfu/m3);
- for Gram-negative bacteria: 1.0×103-2.0×104 cfu/m3 for manufacturing and industrial premises;
- for bacterial endotoxin: 0.005-0.2 µg/m3for productive and industrial premises;
- for fungi: 1.0×101-1.0×104 cfu/m3 for non-industrial workplaces and <1.0×102-1.0×107 cfu/m3 for manufacturing and industrial premises; for the pathogenic microorganisms, there is no safety level (the threshold limit should be 0 cfu/m3).
In special environments like hospital rooms or clean rooms during operation, the threshold limit values for airborne microbial contaminants should be within the range of 1.0×100-4.0×103 cfu/m3 and <1.0×100-1.0×103 cfu/m3, respectively. In case of indoor spaces where a high air quality is required (e.g. clean rooms in action), not only the threshold limit values for biological agents in the air are applied but recommended limits for microbial contamination of surfaces are used as well. A wide review of threshold limit values for occupational bioaerosols can be found in the publications by Brandys & Brandys (2007) or Górny et al. (2011)..
In Europe, Directive 2000/54/EC on the protection of workers against health and safety risks related to exposure to biological agents lays down the principles for the management of biological risks and assigns to employers the duty of assessing the risks posed by biological agents in the occupational environment. The majority of its articles define employers’ obligations regarding: methods of risk determination, assessment, elimination and reduction of risks, rules of hygiene and individual protection, training of workers, records of exposures, accidents and incidents. The classification of biological agents (known to infect humans) into four risk groups, containment measures and levels as well as containment for industrial processes are specified in the annexes. Directive 2000/54/EC gives minimum requirements aiming at reduction of the health risks from biological agents in the workplace. Within the European Union, the Directive provisions have been introduced to the legal systems of individual Member States by suitable legal instruments (e.g. in Poland, it is an Ordinance of the Ministry of Health) or Codes of Practice and guidelines. It is therefore important to refer to the relevant national regulations related to biohazards at workplaces.
Prevention and control measures
Directive 2000/54/EC imposes an obligation on employers to protect workers against risks to their health and safety, including the prevention of such risks, arising or likely to arise from exposure to biological agents at work. When the proper “on site” measurements enable precise recognition of the presence (with its circumstances) of biological agents as well as the nature and possible extent of associated adverse effects, the next step is to design measures to prevent or control these hazards. The best method of preventing adverse health outcomes would be to provide a working environment free from such hazards. While it is not usually feasible to completely eliminate the risk posed by biological agents, its reduction to the lowest possible level can be achieved using collective prevention measures based on the S.T.O.P. principle. This prevention strategy combines four elements, i.e. substitution, technical, organisational and personal measures, with systemic measures in one holistic approach. The examples of safety measures regarding occupational bioaerosol exposure in each of these groups (considering a hierarchy of their importance) are as follows:
- Substitution: elimination or (if not possible) substitution of biological agents/processes with others less hazardous;
- Systemic measures: procedural risk control measures by designing suitable systems of work and maintaining plant and equipment in safe and hygienic conditions (e.g. cleaning of workplaces must be considered an integral part of operations and it should be carried out properly in order to minimise dust generation, workplaces should be designed with easy-to-clean surfaces, separate storage of private and working clothing; scheduled regular cleaning and changing of working and protective clothes, provision of facilities to wash hands (or take a shower) when leaving the workplace, avoidance of eating, drinking or smoking at the workplace, provision of clean and separated storage facilities for food and drinks);
- Technical measures: minimisation of the release of bioaerosols (e.g. fast delivery, short storage times or immediate processing of critical materials), machinery guarding/fencing/shielding and other equipment to reduce bioaerosol immissions, avoidance of manual processing, controlled atmosphere in workplaces with air filtration or air conditioning;
- Organisational measures: isolation of workstations (e.g. automatically closing doors, sluice), restriction of entrance to areas with high bioaerosol levels to an operational minimum number of workers, workers’ information and training to promote safe working habits, medical surveillance (preventive medical check-ups and vaccinations, monitoring of exposure and record-keeping); proper labelling, safe storage, procedures for safe transfer, handling, use and disposal of agents being used;
- Personal measures: respiratory protection including respirators with clean filtered air supply, personal protective equipment (clothes, gloves and goggles; however, such equipment should be used as the last possible prevention measure only when eliminating or reducing the level of risk to an acceptable level is not possible).
New trends in bioaerosol measurement and control
Microbial source strength concept
The first attempt which tries to eliminate all the weak points of the data gathered by traditional bioaerosol sampling is the microbial source strength concept. Source strength in such understanding means the ability of microbial source for maximal emission of particles into the air under the most favourable release conditions. This novel idea assumes a dynamic description of the aerosolisation process of microbial particles from their source (e.g. quantification of the particle emission rate from microbiologically contaminated surface of building material). According to this concept, the aerosolisation potential is not only limited to the emission from the source of fungal or bacterial spores or vegetative cells, but also includes the role of fragments of microbial colony structures. It allows the assessment of maximal potential exposure irrespective of the viability of aerosolised microbial particles or temporal and spatial variations of propagule emission.
Monoclonal antibodies in bioaerosol analyses
Among new techniques, which are currently being developed to analyse bioaerosol samples are a species-specific detection method based on immunoassays using monoclonal antibodies (mAbs) or phage display reagents in combination with enzyme-linked immunosorbent assays (ELISA) or immunofluorescent techniques. A successful implementation of sensitive and specific enzyme immunoassays for fungal aerosols in air quality research might only be achieved if highly specific antibodies are employed, and if sample processing and sample analysis are standardised. This will increase the accuracy and precision of monitoring data and further our understanding of the temporal and spatial dynamics of fungal contamination. Standardisation will also enhance direct comparisons of monitoring results obtained in different laboratories. It is especially important in epidemiological studies characterising causal relationships between adverse health effects and fungal exposure.
Health protection by electric charges and microwaves
Microorganisms in the airborne state carry electric charges. Their amount depends on the microbial species, dispersion method and can be more than 10,000 elementary electric charges. Elaboration of new antistatic materials and subsequent production of special clothes (e.g. medical aprons) may substantially reduce the number of infections caused by airborne microbial agents. As it was shown by Allen & Henshaw (2006), new antistatic apron materials may constitute an improved barrier to the spread of microorganisms exhibiting a 38% reduction in bacteria attracted onto their surface compared with the white plastic aprons currently in use. Ability to carry electrical charges on airborne microorganisms can be also used to eliminate them from the contaminated environment. To reduce the human exposure to bioaerosol particles, the unipolar air ionisation may be successfully applied. As it was shown, a significant percentage (up to 92%) of airborne viable bacteria could be inactivated by the ion emission. Moreover, such process, if performed within the human breathing zone, may considerably enhance protection efficiency of filtering facepiece respirators or even surgical masks. The experiments showed that for particle size range of ~0.04-1.3 μm, air ionisation in the vicinity of a manikin with respiratory mask enables the particle penetration efficiency reduction up to about 3,000-fold.
Due to both high penetration efficiency and sterilisation effectiveness, the microwave technique is utilised for non-invasive cleaning and could help in effective protection against microbial contamination. The experiments revealed that microwave radiation can decrease both fungal and actinomycetal spore viability when growing on building materials. Such radiation can also deprive microbial spores of their cytotoxic properties. The effectiveness of this process, however, strongly depends on the degree of surface hydration and on the combination of power density and time of exposure to microwaves.
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Links for further reading
EU-OSHA - European Agency for Safety and Health at Work 2003, ‘Factsheet 41 – Biological agents’. Available at http://osha.europa.eu/en/publications/factsheets/41/view
EU-OSHA – European Agency for Safety and Health at Work 2011, ‘Factsheet 100 - Legionella and legionnaires’ disease: European policies and good practices’. Available at http://osha.europa.eu/en/publications/factsheets/100/view
Flannigan, B., Samson, R. A. & Miller, J. D. (Eds.), ‘Microorganisms in home and indoor work environments’, CRC Press, Boca Raton, 2011.
Hung, L.-L., Miller, J. D. & Dillon, K. (Eds.), ‘Field guide for the determination of biological contaminants in environmental samples’, AIHA, Fairfax, 2005.
Kolk A., ‘Managing biological hazards in the workplace ’, in Magazine 6, Dangerous Substances Handle with care, EU-OSHA – European Agency for Safety and Health at Work 2003, p. 31
Prezant, B., Weekes, D. M. & Miller, J. D., ‘Recognition, evaluation, and control of indoor mold’, AIHA, Fairfax, 2008.
Waites, M. J., Morgan, N. L., Rockey, J. S. & Higton, G. (Eds.), ‘Industrial microbiology: an introduction’, Blackwell Science Ltd., Oxford, 2001.