Organic dust and gaseous contaminants at wood working shops

Abstract

Airborne dust bioaerosols, ammonia and formaldehyde levels were determined inside two different (ventilated and unventilated) wood working shops. Airborne dust was found at mean values of 4.3 and 3.01 mg m⁻³. These levels were higher than that recommended by Egyptian environmental law [1 mg m⁻³ indoor maximum allowable concentration (MAC) for hard wood]. The highest frequency of aerodynamic size distribution of airborne wood dust was detected at a diametre of 4.9 µm which was recorded during a machining operation. Total viable bacteria were recorded at a mean value of 10⁴ colony-forming units (cfu) m⁻³, whereas Gram-negative bacteria were found at very low counts (10¹ cfu m⁻³). Fungi levels were recorded at mean values of 10³ and 10² cfu m⁻³ in ventilated and unventilated shops, respectively. Penicillium, Aspergillus, Cladosporium and yeast species were dominant isolates. Moreover, actinomycetes were found at a mean value of 10³ cfu m⁻³ at both workshops. Ammonia was detected in relatively low concentrations (mean values of 457 and 623 µg m⁻³), whereas formaldehyde was found in relatively moderate concentrations (mean values of 0.42 and 0.64 ppm).

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Introduction

A number of industrial environments, such as cotton mills, textiles plants, woodshops and sewage plants, contain organic dust and gases, which may cause health hazards. Many investigators^1–3 have found that mucociliary function is impaired in workers exposed to wood dust in the furniture industry for more than 10 years. Acheson et al.⁴ found a relationship between adenocarcinoma of the paranasal sinuses of workers and exposure to wood dust in the furniture industry. The incidence of adenocarcinoma was about 1000 times greater in wood workers than in other people.⁵ Moreover, exposure to wood dust produced pathological changes in the lung,⁶ and certain types of wood produced dermatitis, rhinitis, asthma and allergenic responses.⁷

The exposure to airborne bacteria, fungi, endotoxins and viruses causes potential biological hazards.⁸ Wood and wood products in saw mills, wood working shops and the furniture industry yield both airborne bacteria and fungi.⁹ Saw dust containing different types of fungi causes sequoiosis.¹⁰Aspergillus fumigatus was reported to cause tremorgenic mycotoxins in saw mills.¹¹ Alveolitis and respiratory diseases among wood timber workers were attributed to exposure to Rhizopus microsporus var rhizopodiformis¹² and Penicillium species.¹³ Gram-negative bacteria of plant origin are sources of endotoxins and allergens.^9,14 Rylander¹⁵ suggested that dyspnoea among workers at cotton mills is due to invasion of the airway by leucocytes in response to inhalation of Gram-negative bacteria. A positive relation was found between acute effects of lung function and the level of endotoxins.¹⁶ In addition, Pseudomonas, Klebsiella, Alcaligenes and Acinetobacter are common in organic dust and pose potential hazards to exposed workers.¹⁷

Formaldehyde is found throughout the environment. It originates from many sources, such as incinerators, photochemical smog and engine exhaust.¹ Formaldehyde is used to produce synthetic resins, adhesives, cosmetics, dyes, fibrewood, plastic, rubber, textiles and insulation foam.¹⁸ Irritation of the respiratory tract, eyes and skin is the principal hazard in humans exposed to formaldehyde.¹⁹ Moreover, formaldehyde must be handled as a potential carcinogen²⁰ and as a mutagen.²¹ In contrast, ammonia occurs in body metabolites and in breath, and is a strong toxin to the eyes.²²

The present study aims to evaluate the levels and characters of airborne dust, bioaerosols (bacteria, Gram-negative bacteria, fungi and streptomycetes), ammonia and formaldehyde in work zones of wood working shops, and to determine the population of microorganisms that have potential toxic effects or emit volatile organic compounds.

Materials and methods

Description of wood working shops

Two wood working shops were selected. They differ in area, location, number of workers, load of work and ventilation conditions. The first workshop (located at the National Research Centre) has natural and mechanical (fans) ventilators, whereas the second shop (located at Elharam Street) has neither natural nor mechanical ventilators. Both workshops use a wide range of soft and hard woods. They produce a range of wood products, such as desks, chairs and furniture.

Air sampling

Air samples for dust, microbial and chemical indicators were collected indoors, 2 h after starting work, during working and after stopping of the wood machines. These samples were collected at a height of 1.5 m above ground level.

Dust sampling

The mass concentrations of airborne dust in the work zone were collected on conditioned preweighed cellulose nitrate filter membranes (pore size, 0.45 µm; diameter, 25 mm). Sequential half-hour samples were obtained using an open face holder and sampling pump calibrated to draw 5 l min⁻¹. The filters were conditioned in a desiccator before weighing and the concentrations of airborne dust was calculated in mg m⁻³.

Microbial analysis

Air samples were collected using a slit sampler (Model TVπ 818N^o 5587, CAEπAHO B CCCP) at a flow rate of 25 l min⁻¹. The sampling periods ranged between 1–4 min. Trypticase soya agar, 3% malt extract agar, MacConkey agar and starch casein agar (Difco, Detroit, MI) media plates were used for the counting of total viable bacteria, fungi, Gram-negative bacteria and actinomycetes, respectively. Bacteria/environmental plates were incubated at 25–30 °C for 48 h, whereas Gram-negative bacterial plates were incubated at 37 °C. Fungi and actinomycetes plates were incubated at 28 °C for 7 days. Fungi isolates were identified microscopically, whereas bacterial isolates were identified according to Bausum et al.²³ The aerodynamic diameter (ad) of the dust and fungi spores was calculated from the density, shape and physical diameter of the particles (physical diameter measured microscopically).

Chemical analysis

Sequential 1 h air samples for formaldehyde and ammonia were collected using a pump calibrated to draw 1 l min⁻¹ at the breathing zone. The indoor air concentration of total aliphatic aldehyde (as formaldehyde) was determined using the 3-methyl-2-benzothiazolone hydrazone hydrochloride (MBTH) method.²⁴ Ammonia levels were determined using dilute sulfuric acid as an absorbing reagent, followed by reaction with Nessler's reagent and colorimetric estimation.²⁵

Statistical analysis

The linear correlation coefficient (r) and Student's t test (p = 0.05) were used to examine the relationships between viable agents and airborne dust and gases.²⁶ Logarithmic transformation (log x + 1) was used to normalize microbial data.

Results

A total of 14 trials were carried out at both workshops. High airborne levels were found at both wood workshops. The highest concentration was found during a machining operation (8.9 mg m⁻³), whereas the lowest was detected when machines were inoperative (0.65 mg m⁻³) (Table 1). The aerodynamic diameters of airborne wood dust ranged between 1.2 and 20 µm. The highest frequency (17%) of aerodynamic size distribution was 4.9 µm, which was detected during a machining operation. Moreover, phase contrast microscopy showed that dust particles are fibrous, flaky and cylindrical in shape.

Table 1 The range and mean concentrations of air indicators at two wood working shops

	Ventilated workshop (site 1)		Unventilated workshop (site 2)
Indicator	Range	Mean	Range	Mean
TVBC, total viable bacterial count; Actin, actinomycetes.
Suspended particulate/mg m^−3	0.65–8.928	4.307	0.66–8.930	3.01
TVBC/cfu m^−3	3.1 × 10^3–2.3 × 10^4	1.3 × 10^4	6.9 × 10^3–3.5 × 10^4	1.39 × 10^4
Gram-negative/cfu m^−3	0–3.12 × 10^2	7.0 × 10	1.6 × 10–6.6 × 10	3.8 × 10
Total fungi/cfu m^−3	9.2 × 10^2–3.5 × 10^3	2.3 × 10^3	8 × 10–3.6 × 10^2	2.01 × 10^2
Actin/cfu m^−3	6.4 × 10–8.23 × 10^3	3.36 × 10^3	0–6.78 × 10^3	1.72 × 10^3
Ammonia/µg m^−3	300–630	457	528–714	623
Formaldehyde/ppm	0.28–0.54	0.42	0.48–0.84	0.64

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Bioaerosol concentrations

Total viable bacterial counts ranged between 10³ and 10⁴ colony-forming units (cfu) m⁻³, with a mean value of 10⁴ cfu m⁻³ in both workshops (Table 1). Gram-negative bacteria were found in the range 0–3.12 × 10² cfu m⁻³ (with a mean value of 70 cfu m⁻³) and 0–66 cfu m⁻³ (with a mean value of 38 cfu m⁻³) in ventilated and unventilated workshops, respectively (Table 1).

The identification of bacterial isolates is summarized in Table 2. Gram-positive bacteria (cocci and bacilli) constituted 92.85%, whereas Gram-negative bacteria comprised 7.15% only. Bacillus, Diplococcus, Staphylococcus and Micrococcus were dominant, whereas Sarcina and Tetrads were isolated but in lower counts. Acinetobacter was the dominant Gram-negative bacteria, and Alcaligenes, Flavobacterium and Pseudomonas were also isolated (Table 2).

Table 2 Identification of bacterial isolates

Type	No.	%
Gram-positive
Diplococcus	54	17.53
Micrococcus	36	11.69
Staphylococcus	44	14.28
Sarcina	36	11.69
Tetrads	16	05.2
Bacillus	100	32.46
Gram-negative
Acinetobacter	10	3.25
Alcaligenes	4	1.3
Flavobacterium	6	1.95
Pseudomonas	2	0.65
Total isolates	308	100

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Fungi levels averaged between 9.2 × 10² and 3.5 × 10³ cfu m⁻³ and between 80 and 3.6 × 10² cfu m⁻³ in ventilated and unventilated woodshops, respectively (Table 1). The mycological examination of 222 isolates is recorded in Table 3. Penicillium spp., Aspergillus spp., Cladosporium and yeasts were the predominant fungi. Alternaria, Helminthosporium, Mucor, Rhizopus, Spicaria, Fusarium and Scopuloriopsis were found, but in lower counts. Penicillium spp., Aspergillus niger, Asp. versicolor, Cladosporium and yeasts were found in higher counts in the ventilated workshop than in the unventilated one. In contrast, Asp. flavus, Spicaria and Scopuloriopsis were detected in the unventilated workshop only.

Table 3 The predominant fungi genera and their physical and aerodynamic diameters^a

	Site
Type	No. 1	No. 2	Physical diameter/µm	Aerodynamic diameter/µm
^a —, not detected.
Penicillium spp.	75	25	1.7–3.5	1.6–3.1
Aspergillus niger	17	1	3.5–4.9	3–4.5
Asp. versicolor	18	—	2.5–3.5	2–3.5
Asp. flavus	—	6	2.5–3.5	2.3–3.1
Cladosporium spp.	25	1	(2.8–5) × (5–14)	2.5–4.5
Alternaria	2	1	(7–10) × (10–17)	6–10
Helminthosporium	1	—	(4.6–10) × (9–17)	4.6–9
Mucor	4	—	10–14	10–12
Rhizopus	2	1	4.9–14	4.6–12
Spicaria	—	1	2.5–4.9	2.4–4.5
Scopuloriopsis	—	2	3–4	2.5–3.5
Fusarium	1	—	(2–3.1) × (4.6–17)	2–3.1
Yeasts	32	3	2.8–3.5	2.8–3.8
Non-sporulating mycelia	4	—	—	—

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Table 3 shows the physical and measured aerodynamic diameters of the fungal isolates. The aerodynamic size is a critical factor for evaluating respiratory exposure to fungal spores. Penicillium, Aspergillus spp., Cladosporium (short axis diameter), Spicaria and Scopuloriopsis have aerodynamic diameters (ad) of less than 5 µm, and can enter the gas exchange tissues of the lung. On the other hand, Helminthosporium, Mucor, Alternaria and Rhizopus have aerodynamic diameters greater than 5 µm, and may be deposited in the nasal region.

Actinomycetes (streptomycetes) were found in relatively high counts (a mean value of 10³ cfu m⁻³) in both workshops. Streptomycetes spores are well suited for deep penetration into the lung on inhalation (ad between 1 and 1.5 µm).

Gas concentrations

Ammonia was found at relatively low levels in both workshops. It was detected at a relatively higher mean value, 623 µg m⁻³, in the unventilated workshop than in the ventilated workshop (a mean value of 457 µg m⁻³). Formaldehyde levels ranged between 0.28 and 0.84 ppm. It was detected at mean values of 0.63 ppm and 0.42 ppm in the unventilated and ventilated workshops, respectively (Table 1).

Table 4 shows the correlation coefficients (r) between bioaerosols and airborne dust and gases. There are positive and sometimes significant relationships between bacterial and actinomycetes counts and airborne dust, ammonia and formaldehyde (Table 4). In contrast Gram-negative bacteria were insignificantly and negatively related with airborne dust, ammonia and formaldehyde. A positive relation (r = 0.37) and a negative relation (r = −0.53) were detected between fungi and airborne dust in ventilated and unventilated workshops, respectively. Moreover, negative relations were recorded between fungi and gases in both shops.

Table 4 The correlation coefficients between bioaerosols and suspended particulate and gaseous indicators at the two wood working shops

	SPM		NH3		HCHO
Agent	Site 1	Site 2	Site 1	Site 2	Site 1	Site 2
^a Significant (p = 0.05). SPM, suspended particulate matter; TVBC, total viable bacterial count; Actin, actinomycetes.
TVBC	0.91^a	0.89^a	0.75	0.74	0.81	0.74
Total fungi	−0.53	0.37	−0.51	−0.1	−0.197	−0.022
Gram-negative	−0.36	−0.24	−0.55	−0.21	−0.62	−0.181
Actin	0.87^a	0.94^a	0.73	0.78	0.92^a	0.73

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Discussion

There are few publications concerned with the levels of airborne dust, aerosols and gases inside wood working shops. A wide range of values of airborne dust were obtained in the present study. The detected levels were higher than the industrial maximum allowable concentration (MAC) recommended by the Egyptian Environmental Affairs Agency (EEAA)²⁷ (1 mg m⁻³ for hard wood). Hounam and Williams²⁸ found airborne dust in the range 1–25 mg m⁻³, with a median of 4.2 mg m⁻³ and a mean value of 5.9 mg m⁻³, in five large wood factories. Al-Zuhair et al.²⁹ found airborne dust at an average of 0.72–4.4 mg m⁻³ in a wood working zone. The coarsest dust size was produced during machining operations, whereas the finest was detected when machines were inoperative due to the removal of the coarser fractions by gravity. The dust samples of the ventilated workshop (site 1) were greyer in colour than those of the unventilated workshop (site 2). This indicated that general atmospheric pollution rather than wood dust had infiltrated from outdoors to indoors. Wood particles are fibrous and flaky in character and have a large area, which could facilitate the leaching of soluble toxic constituents.³⁰

In the present study, total viable bacterial counts exceeded the average (3.4 × 10³ to 10⁴ cfu m⁻³) recommended for indoor environments.³¹ However, Gram-negative bacteria were found in low counts. Airborne Gram-negative bacteria were detected at a mean concentration of 33 cfu m⁻³ in a saw mill and ranged between 870 and 1020 cfu m⁻³ at a large wood furniture factory.²⁹ Gram-positive (cocci and bacilli) and Gram-negative (Acinetobacter, Alcaligenes, Pseudomonas and Flavobacterium) types were dominant isolates. Gram-positive cocci (Staphylococcus and Micrococcus) are normally predominant indoors.³² However, the presence of Gram-positive bacteria indicates overcrowding and inadequate ventilation, whereas Gram-negative bacteria indicate the presence of a contamination source.³³Acinetobacter calcoaceticus, Gram-negative and coccoid bacteria are common in airborne dust, and cause pulmonary infections.³⁴ Moreover, Bacillus species are numerous in organic dust and are related to allergic alveolitis.³⁵ Milanowski³⁶ found cocci in high counts in dusts of plant origin.

The observed fungi counts are in agreement with Al-Zuhair et al.,²⁹ who detected fungi at a mean concentration of 1.1 × 10³ cfu m⁻³ at a wood workshop. Fungi were recorded in higher counts in the ventilated than in the unventilated workshop due to the large release of spores by the effects of the air current.³⁷Penicillium and Aspergillus species were dominant in both workshops in the present study. Aspergillus and Penicillium are normal indoors, whereas Cladosporium and Alternaria are of outdoor origin.³⁸ In the present study, Cladosporium and yeasts were detected in relatively high counts in the ventilated workshop. This indicates that the outdoor air contaminants are infiltrated indoors. Reponen³⁹ found Cladosporium and yeasts in higher counts outdoors than indoors. Moreover, Penicillium, Aspergillus, Cladosporium, Fusarium, Mucor and Rhizopus were dominant in cotton mills,⁴⁰ while Penicillium and Rhizopus were dominant in wood workshops.¹¹ Aflatoxin inhalation was reported as a problem for workers exposed to corn and peanut dusts,⁴¹ and mycotoxins are produced by Penicillium and Aspergillus.⁴²

The relatively high counts of streptomycetes may be due to their presence at large levels in dusts of plant origin. Their presence in indoor environments is an indication of contamination.³³ Actinomycetes are important air contaminants in agriculture and waste composting plant.⁴³ Several types of actinomycetes are associated with allergenic alveolitis.⁴⁴ Also, streptomycetes species stimulate lung macrophage reactions, which lead to inflammation and tissue injury.⁴⁵

In the present study, formaldehyde was detected at relatively moderate levels. Volatile metabolites produced by fungi can cause a musty odour in mouldy buildings.⁴⁶ Aldehyde, butanol, short chain alcohols and 1-octen-3-ol-but-ethylacetate are dominant volatile materials produced by fungi.¹⁰ In contrast, ammonia was detected at low levels. The threshold limit of ammonia is 25 ppm.²⁷ Continued exposure to formaldehyde at concentrations ranging between 0.1 and 0.5 ppm is irritating to humans.²⁰ However, a very low occupational threshold level of formaldehyde (0.75 ppm) is recommended in the USA (value given by the Occupational Safety and Health Administration, OSHA) and 0.5 ppm in Germany.⁴⁷

The positive relationships between the viable fraction (bacteria and fungi) and airborne dust may be due to the fact that the viable fraction is mainly associated with large dust particles.⁴⁸ Natural volatile gases from vegetation, such as α-pinene and terpenes from shrubs and herbs, act as aerial disinfecting agents against air microorganisms.⁴⁹ Reynolds et al.⁵⁰ found a negative relation between Gram-negative bacteria and NH₃. Formaldehyde, CO, and O₃ are known to damage cell wall membranes.⁵¹ However, the outer sheath of streptomycetes is protected against physical damage and drying.⁵²

Conclusions

(1) Airborne dust varied between 0.66 and 4.4 mg m⁻³, and the highest frequency of dust size distribution was 4.9 µm, during machining operations.

(2) High levels of fungi, bacteria and actinomycetes were recorded, whereas Gram-negative bacteria were found in low counts.

(3) Low levels of formaldehyde and ammonia were detected, and long exposure to these gases can be very dangerous.

(4) Most of the wood workers are exposed to a complex biological and chemical mixture.

(5) Chemical gas may preserve or kill airborne organisms.

(6) The present levels of bioaerosols, airborne dust and gases are an indication of the inadequate ventilation and overcrowding of both shops.

(7) An occupational hygiene study must be carried out to assess the potential health risks arising from exposure to airborne contaminants in the wood working shops under investigation.

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