Introduction 

Motor-manual tree felling and processing (i.e., by chainsaw) is still very common in many countries ([37], [39], [7], [3], [48]). Motor-manual forest operations are inherently dangerous ([49], [32], [47]) and cannot benefit from the safety improvements offered by high mechanization ([5]). Steep terrain, ownership fragmentation and close-to-nature management criteria slow down the introduction of mechanized harvesting in mountainous conditions ([46]). Workers engaged in forest cutting who use chainsaws are exposed to noise and vibration stresses and to the hazardous effects of exhaust gases as well as floating particles of mineral oil and airborne wood dust ([38], [25]).

Potential health effects from exposure to wood dust have been studied and include pulmonary function changes, allergic respiratory responses (asthma) and cancer of nasal cavity and paranasal sinuses. The irritant effects of wood dust are well documented ([44], [50], [20], [22]). Respiratory, nasal and eye symptoms are the most common effects reported by woodworkers ([17], [31], [40], [45], [33]). However, not all studies agree. A recent US study has shown fewer or no symptoms from typical exposures ([13]). Other studies have addressed the relationship between exposure to wood dust and skin pathologies ([23]) or asthma ([16], [35]). The most serious problem araising from wood dust exposure is the risk of developing cancer, mainly nose and sinus adenocancer ([41], [29], [28]). Nasal cavity adenocancer was diagnosed much more frequently in woodworking industry operators (saw mill, joinery, furniture, etc.) than in the rest of the human population, where this malignant disease is very rare and only accounts for 0.25% ([15]). Hausen ([15]) also pointed out that wood chemical components can have serious biological effects on human health even at low concentrations, if long-term exposure occurs.

The International Agency for Research on Cancer (IARC) classified hardwood dust as a human carcinogen ([19]), estimating that at least 2 million people worldwide are exposed to the noxious effect of wood dust. According to the dimension of component particles (International Organization for Standarization - [24]), wood dust can be classified into inhalable, thoracic and respirable dust ([1]). According to the Scientific Committee on Occupational Exposure Limits (SCOEL) recommendation, the inhalable fraction “is the best convention to explain the critical effect(s) of wood dust in the upper airways and it would therefore be the most appropriate fraction to sample” ([43]).

In 1999, the European Union published Directive 99/38/EC, setting the legal limit for the exposure to inhalable wood dust at 5 mg m^-3, as an average of a 8-h working day ([10]). This limit is defined as occupational exposure limit (OEL) and is valid for exposure to hardwood dust or to any mix of hardwood and softwood dust. This OEL is not applied for exposure to pure softwood dust, which is not yet a legally recognized as a noxious substance. The EU OEL, was confirmed in Directive 2004/37/EC ([11]) and is applied in Italy and Finland. In countries like Spain and the United Kingdom, the OEL is the same but includes both hardwood and softwood inhalable dust. In still other countries, the OEL, referred to inhalable fraction, is lower and usually without distinction between softwood and hardwood: 3 mg m^-3 in Belgium, 2 mg m^-3 in Austria, Germany and Sweden, and 1 mg m^-3 in France. Symptoms in the upper respiratory system have been reported also at much lower exposure levels, from 1 mg m^-3 ([12]).

In 2003, SCOEL suggested applying a lower value between 1 and 1.5 mg m^-3, without distinction between softwood and hardwood ([43]). Moreover, in 2012, the Advisory Committee on Safety and Health at Work ([2]) of the European Commission proposed amendment of Directive 2004/37/EC, including an OEL for wood dust of 3 mg m^-3, measured as inhalable dust, with a review period of 3-5 years.

In United States, the American Conference of Governmental Industrial Hygienists ([1]) and the National Institute for Occupational Safety and Health (NIOSH) set a Threshold Limit Value (TLV^®) of 1 mg m^-3 for most wood species, without distinction between softwood and hardwood, or lower for the western red cedar (0.5 mg m^-3 - [30], [8]).

In relationship to wood dust exposure and its effect on health, many studies have been carried out taking into consideration woodworking industry workers (sawmill, joinery, etc.). Moreover, epidemiological studies have examined exposure to wood dust deal in the furniture industry, which employs many workers and is much easier to reach ([4]).

Even though it is well known that the working environment in logging operations can be dusty ([36]), very few studies have addressed forest operators’ exposure, mainly taking into account the respirable wood dust fraction in chainsaw operation ([18], [25]) or chipping operation ([34]).

To fill the gap in knowledge and have a comprehensive framework on the exposure of forest workers to wood dust, field surveys during motor-manual felling and processing of trees (i.e., with chainsaw) were carried out in central Italy. The objectives of this study were to evaluate exposure to inhalable wood dust among forest workers and highlight significant differences among: (i) different silvicultural treatments (clear cut in coppice and thinning, pruning and sanitary cut in high stand); and (ii) chainsaw fuel. In addition, the different tasks performed by the workers were timed to highlight relationship between wood dust concentration and chainsaw running time.

  Materials and methods 

All study areas were located in Tuscany, on the Apennine mountain range. Four silvicultural treatments were considered (Tab. 1).

(i) Clear cut in coppice with standards in two pure stands and one mixed stand. Coppice forests represent about 60% of the total forest area of Italy ([21]). Coppicing operation consists of cutting all the shoots growing from suckering stumps, leaving only standards (30-60 per hectare, depending on species). Shoots were felled, debranched, and cross-cut into 1-metre length logs by chainsaw, then the logs were more finely cleaned of twigs using a billhook and were manually piled. The main assortment obtained was firewood. During data collection, the operators worked singly at a safe distance from each other.

(ii) Thinning from below in two mixed stands and one pure stand. This operation consisted of removing a percentage of the trees (25-30%) to improve growing conditions. Usually, small, badly formed or failing trees were cut. Trees were felled, debranched and cross-cut into 5 to 6 metre length logs by chainsaw. The operators worked singly at a safe distance from each other.

(iii) Sanitary cut in two pure stands and two mixed stands. This silvicultural treatment consisted of removing dead, damaged or diseased trees to avoid spread of parasites and to prevent forest fires. An operator with chainsaw felled and processed the trees to obtain logs of 5 to 6 metre length. The operators worked singly at a safe distance from each other.

(iv) Pruning in three pure stands and two mixed stands. Pruning consisted of removing the lower dead branches of live trees by chainsaw, up to a height of around 2 m. Dead or uprooted trees were also felled and cut into logs. The operators worked singly at a safe distance from each other.

In total, the study included 100 forest operator working days: 20 in coppice clear cut, 28 in pruning, 23 in thinning and 29 in sanitary cut.

All the forest operators who performed the activity had long experience in this kind of felling operations. During the study, the workers used their usual chainsaws (Tab. 2). All the chainsaws were in good condition and carefully maintained.

Three different fuels were used during the study: normal two-stroke gasoline mix (NG, a mixture of 2% oil and lead-free gasoline) and two alkylate fuels (Alk1 and Alk2, as usual already mixed with motor lubricating oil) sold by two major international chainsaw manufacturers. Each operator used only one type of fuel during the same sampling day.

To collect inhalable fraction of wood dust, during chainsaw operation, each forest workers wore a SKC Button Sampler with binderless fibreglass membrane (Sartorius) of 25 mm in diameter (Fig. 1).

Fig. 1 - Personal sampler. SKC Button Sampler on the operator’s jacket (left) and Gilian 5000 portable pumps (right).

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The sampler was made of steel with a semi-spherical protective shield with conical micro-holes to avoid aspiration of non-inhalable projectile particles ([9], [14], [30]). Inclusion of these large particles would bias the sampling because they are too heavy to be inhaled ([27]). Furthermore, this multiorificed inlet reduces sensitivity to wind direction and velocity ([26]).

The sampler used for the study was connected by a transparent flexible tubing to a Gilian 5000 portable pump (Fig. 1). The SKC Button Sampler operated at a flow rate of 4 l min^-1.

The pump was calibrated at the start of each day of sampling using a flow meter (Gilian Challenger). The pump recorded the total air flow and the duration of the sampling session. The portable pump was attached to the belt on the operator’s back, and the sampler was placed at a distance of 10 cm from the operator’s face, i.e., at lapel height on the right side of operators’ jackets (Fig. 1).

Daily dust exposure was then determined by a gravimetric method. Before the tests, filters were conditioned in a climatic cabinet (Activa) set at a temperature of 20 ± 1 °C and moisture of 48 ± 2 % for 24 hours together with three control filters. The filters were then weighed in the laboratory with a precision scale accurate to the microgram (Sartorius ME36S^®) and placed into sealed boxes identified with code numbers. Before starting each test, a filter was carefully placed into the sampler using clean tweezers to avoid contamination. At the end of the tests, filters were removed with tweezers and placed back in their respective coded boxes. These were sent to the laboratory, where used filters were reconditioned for 24 hours in the same climatic cabinet. After conditioning, filters were weighed again with the same scale together with the three control filters conserved in sealed boxes at the laboratory.

Finally, the concentration of wood dust was measured using the following formula (eqn. 1):

where C is the wood dust concentration in mg m^-3; P2 is the weight of the filter after the test in mg; P1 is the weight of the filter before the test in mg; and V is the air volume in m³, calculated as (eqn. 2):

where T is the duration of the sampling in minutes and F is the effective air flow in l min^-1.

If the average value of the differences in weight of the control filters (weight after - weight before) was ≠ 0, the average difference was added (if <0) or deducted (if >0) from wood dust weight.

Each sampling lasted the length of the work shift and ranged between 6 and 8 hours. Sampling data was expressed as a time-weighted average (TWA) over 8 hours.

At each work site, dust monitoring was personally supervised by the researchers, who also checked the proper running of the pumps and the correct position of the devices.

Timing of work tasks

Work time was split into time elements ([6]), recorded separately for every worker involved in these tasks to identify the incidence of chainsaw running time on gross time. We determined the various time elements of the work, with special attention to recording the duration of chainsaws’ running and idling time, i.e., potentially producing wood dust. During data analysis time elements were separated into: (i) chainsaw running time, including time for felling, branch removal, crosscutting, stump tidying (if necessary), moving about on site; (ii) other productive time, including time used to perform bill hook or axe tasks, evaluation of plants, moving about on site; (iii) time for transfer, including time for travelling to and from the site, if included in working hours; (iv) preparation time, including time for preparing and putting away tools; and (v) delays (refuelling, maintenance, sharpening, pauses, setbacks and other non-working events).

Working time was recorded using a chronometric table with centesimal (1 min = 100 cmin) stopwatches.

Statistical analysis

The data were entered in a data sheet and analysed using the R open-source software (R Development Core Team, Wien, Austria - ⇒ http:/­/­www.­r-project.­org). Correlations relevant to the aims of the study were sought, i.e., the relationships between the variables measured (chainsaw running time, wood dust), the type of silvicultural treatment and fuel type. Normal distribution of the variables was checked by the Lilliefors test and homoscedasticity (homogeneity of variance) by Levene test. Wood dust exposure data were logarithmically transformed with the base 10 (i.e., for wood dust in operation type and wood dust in fuel type) due to the non-normal distribution. One-way ANOVA was then used to calculate mean square error for Tukey’s HSD test. By comparing pairs, this test revealed statistically significant differences between the means.

Chainsaw running time differences in relationship with silvicultural treatment were tested with the Kruskal-Wallis multiple comparison test due to the non-normal distribution of data. For this data, we did not find a satisfying normalization function, so we prefer to apply a non-parametric method.

  Results 

Working time

Tab. 3 summarises the distribution of the working time in the considered phases.

“Chainsaw running time” showed a statistically significant difference between clear cut in coppice with standards and the other silvicultural treatments (p < 0.001 - Tab. 4). In particular, coppicing with standards showed the lower chainsaw running time, while no difference was recorded among the treatments performed in high forest stands. This was expected because coppicing involves many tasks, that do not involve chainsaw use.

The highest value was recorded in pruning (Tab. 4), since conifer pruning does not involve any particular assessment of plants or use of other tools, as in felling, and the chainsaw is used continuously for longer periods.

A higher standard deviation suggests that the chainsaw running time during thinning varied more than during the other silvicultural treatments.

Wood dust

Wood dust response to type of silvicultural treatment

Tab. 5 shows that only 2 samples (2%) exceeded the European OEL (5 mg m^-3). One of these samples (1%) was recorded in pruning, which is a typical treatment for conifers only, i.e., softwood dust that at present is not included in the OEL. Tab. 5 also shows the exceedances for the lower limits applied in some other European countries and the United States: less than 10% exceeded the limit of 3 mg m^-3, and more than 50% exceeded 1 mg m^-3.

The mean exposure to wood dust during coppicing was significantly higher than during thinning and sanitary cut, while pruning did not show statistical differences with the other treatments (p = 0.002 - Tab. 6).

Wood dust response to type of chainsaw fuel

To check whether the type of fuel used in the chainsaw affected the wood dust exposure, a one-way ANOVA was performed. The analysis did not show any statistical differences among the types of fuel used (p = 0.253 - data not shown). However, the normal fuel showed slightly higher values of mean wood dust concentration. This was probably due to the presence of unburned particles of gasoline or lubricant during combustion.

  Discussion 

Very few studies have tried to determine the exposure of forest operators to wood dust ([18], [25], [34]), probably because of the relatively small population and the difficulty of organizing field tests in the forest ([12]).

The results of our study provide important indications about the exposure of forest workers to wood dust during motor-manual felling. The values of wood dust were considerably below the EU OEL in 98% of cases. The means were about 1.5 mg m^-3 for all operations except coppicing, which showed a mean value significantly higher. In detail, clear cut in coppice showed the highest average exposure to wood dust, and one sample exceeded the EU OEL of 5 mg m^-3. Moreover, 10% of the data recorded were higher than 3 mg m^-3, and only 5% of the data recorded were lower than 1 mg m^-3 (Tab. 3, Tab. 4, Tab. 6). These results contrast with the chainsaw running time recorded in clear cut in coppice, which was significantly lower than in the other silvicultural treatment, thus suggesting a lower wood dust exposure.

In pruning operation, one sample exceeded the EU OEL, 11% of the data recorded were higher than 3 mg m^-3, and only 25% of the recorded data were lower than 1 mg m^-3. Moreover, if we consider the OEL applied in other countries, which are usually lower than the EU OEL, the recorded exposures highlighted critical situations. The results recorded in clear cut in coppice and in pruning may be explained by these facts: (i) In coppicing mainly hardwood species are cut, and this may cause a higher production of wood dust compared with softwood cutting ([19], [42]); and (ii) in coppicing and pruning, it is quite common for the operator to have his/her face very close to the cutting area when the bottom of the guide bar is used for cutting. When using the bottom of the guide bar, the chain is running towards the operator, throwing shavings and dust against him/her, thus increasing exposure to wood dust. Further studies are required to support this hypothesis and explain why in coppicing and pruning higher wood dust exposure was recorded.

The lower average wood dust exposure was recorded in sanitary cut, for which all the samples showed values lower than 3 mg m^-3 (Tab. 5). These results were likely affected by the wood condition, frequently decayed (i.e., lower cutting area because of heart rot) and/or extremely wet (i.e., because of wetwood), which reduced the amount of dust production during cutting.

About half the data recorded in thinning were lower than 1 mg m^-3, and about 9% of the samples were included in the range 3 to 5 mg m^-3. The better conditions in terms of wood dust exposure were likely due to the type of wood that was easily cut by the chain teeth with a lower production of fine particles. However, further and specific studies are required to highlight the effect of the plant species on the production of wood dust in chainsaw cutting.

The type of fuel did not affect the cutting performance and the exposure of forest workers to wood dust.

The other few studies on wood dust exposure of forest workers during chainsaw operations were carried out in Croatia ([18], [25]). However, respirable and not inhalable wood dust data were recorded during these studies, and thus the results are not comparable with the results of our study. Horvat et al. ([18], [25]) recorded values lower than 1 mg m^-3 for respirable dust both in fir wood and in oak wood cutting and processing operation with chainsaws. In Croatia, according to the proposal of the Regulatory Act on maximum permissible concentrations (MPC) of hazardous substances in the working atmospheres and biological limit values (BLV), maximum permissible concentration of wood dust of hardwood species (beech and oak) at the workplace is 1 mg m^-3 for respirable particles and 3 mg m^-3 for total dust.

  Conclusion 

According to our findings, the exposure of forest workers to wood dust was usually lower than the EU OEL, even though 2 samples exceeded that standard. Nevertheless, the average values recorded were close or higher than the OEL applied in some countries (e.g., 2 mg m^-3 in Austria, Germany and Sweden, 1 mg m^-3 in France), and higher or included in the exposure range values suggested for the future by the SCOEL (1-1.5 mg m^-3).

However, in considering our results, it is important to highlight that at present, the OEL are set on the basis of studies of the woodworking industry. This means that the EU and national laws are at present designed to be effective in an industrial environment and they are probably not suitable for evaluating forest operation in the field, where additional variables affecting dust exposure and its effects on workers’ health are not yet defined and assessed. A constructive criticism of the current risk assessment and OEL, designed for an industrial environment, is that they are based on labour carried on for 8 hours a day and around 200 days a year. It should be recalled that the average exposure to wood dust of forest workers is usually lower (<100 days per year) and that their overall working-life exposure is different from that of woodworking industry workers.

Specific epidemiological studies on forest operators should be developed in different countries to examine the relationship between chainsaw operations (i.e., wood dust exposure) and cancer (e.g., nasal cavity and paranasal sinuses cancers) or other occupational diseases.

The first results provided by this study represent a broad and valid database on exposure of chainsaw workers to wood dust. However, further studies are strongly recommended. Future developments on this topic should be: (i) to verify whether the highest values are significant and representative of a particular type of work or species under cutting, or whether they can be ignored; (ii) to investigate if different types of use of chainsaws may affect wood dust exposure (e.g., reducing as much as possible the use of the bottom of the guide bar); and (iii) to review the law to ensure well designed and prudent analysis of real working conditions in forests.

  Acknowledgements 

During the preparation of this paper, our colleague and friend Gianfranco Sciarra passed away. Gianfranco was a researcher who passionately addressed different aspects and issues of industrial hygiene. This paper is dedicated to his memory.

This study was carried out within the project promoted by the Tuscany Region in 2011 as a new research project on the evaluation of forest operators’ hardwood dust exposure in chainsaw cutting operation using a standardized survey methodology. The authors would like to thank the Tuscany Region, the Territorial Office for Biodiversity of Vallombrosa (Corpo Forestale dello Stato), the Union of Municipalities of Casentino and of Valdarno/Valdisieve for their assistance in providing forest sites, forest operators and equipment and tools used in the field operations.

Individual contributions of authors to the manuscript: EM conceived the study and checked the final version of the manuscript; FN carried out the field measurements and draft the manuscript, MC carried out the field measurements; AL carried out the field measurements and performed the statistical analysis; CF performed the statistical analysis; GS carried out the laboratory analysis; FF carried out the field measurements.

This study was funded by the Tuscan Regional Administration.

  References