ISSN e: 2007-4018 / ISSN print: 2007-3828

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Vol. XXVI, issue 2 May - August 2020

ISSN: ppub: 2007-3828 epub: 2007-4018

Scientific article

Analysis of the physical work environment in sawmills in El Salto, Durango, Mexico

http://dx.doi.org/10.5154/r.rchscfa.2019.04.035

Silva-Lugo, Edwin D. 1 ; Aragón-Vásquez, Alondra Y. 1 ; Nájera-Luna, Juan A. 1 * ; Hernández, Francisco J. 1 ; Cruz-Carrera, Ricardo de la 1 ; Carrillo-Parra, Artemio 2

  • 1Tecnológico Nacional de México-Instituto Tecnológico de El Salto (ITES), Programa de Ingeniería Forestal. Mesa del Tecnológico s/n. C. P. 34942. El Salto, Pueblo Nuevo, Durango, México.
  • 2Universidad Juárez del Estado de Durango, Instituto de Silvicultura e Industria de la Madera (UJED-ISIMA). Bulevar del Guadiana núm. 501, Ciudad Universitaria. C. P. 34120. Durango, México.

Corresponding author: jalnajera@itelsalto.edu.mx; tel.: +52 618 158 7940.

Received: April 22, 2019; Accepted: January 24, 2020

This is an open-access article distributed under the terms of the Creative Commons Attribution License view the permissions of this license

Abstract

Introduction:

Sawmill workers carry out their work in an adverse physical environment that influences their well-being, not knowing if safety levels in the area are acceptable.

Objective:

To analyze workers' exposure to noise, thermal comfort, and lighting at five workstations in six sawmills in the El Salto region of Durango, Mexico.

Materials and methods:

The physical variables of the work environment were measured directly at three times of the day for seven workdays at five workstations (forklift, edger, swing saw, resaw, and head saw). The maximum allowable time of exposure to noise, thermal comfort through effective temperature, and lighting of the work area were determined. Statistical differences in physical variables between workstations, sawmills and times of the day were detected by analysis of variance and Kruskal-Wallis median range comparison tests.

Results and discussion:

The noise level (85 to 102 dB[A]) represents a greater hearing risk at the head saw, resaw and swing saw workstations as it exceeds 90 dB (A). Thermal comfort was 20 °C, which ensures that the worker can be exposed 100 % of the time of the workday to this temperature. Lighting levels were high, exceeding 2 000 lx.

Conclusions:

The noise and lighting levels are high in the assessed sawmills and pose a risk to the safety of workers if the mandatory use of ear and eye protectors is not monitored.

Keywords:Thermal comfort; lighting; noise; job safety; forest worker

Introduction

Forest workers in sawmills are generally exposed to adverse conditions that directly influence well-being (Morabito et al., 2014). These conditions act in the long term to gradually undermine health, especially when workers are exposed to high levels of noise, particles, toxic gases, variations in temperature and humidity, inappropriate light, inadequate postures and vibrations (Alves et al., 2002). These factors cause discomfort and increase the risk of accidents and cumulative trauma injuries; that is, the worker will only perceive the negative effects after a few years of exposure to a working condition that he or she initially considered comfortable (Fiedler et al., 2009).

Ergonomics is defined as the application of knowledge of human characteristics to the design of systems. In a production system, people operate within an environmental setting; in this sense, environmental ergonomics can be described in terms of thermal sensations, noise level and light (Alves et al., 2002; Parsons, 2000) that when they exceed certain limits can cause discomfort and harm to workers' health (Reis-Dutra, Pinto-Leite, & Dutra-Massad, 2012).

In the workplace, microclimate-related effects are connected to environmental variables that affect the heat exchange between man and the environment, making thermal comfort difficult to achieve, as the worker often operates in different thermal sensation conditions (Marucci et al., 2013). In hot environments, the worker is prone to dehydration, cramps, exhaustion and heat stroke (Bates, Parker, Ashby, & Bentley, 2001), and in cold environments, to loss of sensitivity, reaction and mobility of extremities, enabling the occurrence of accidents (Blombäck, 2001; Mäkinen & Hassi, 2009).

Noise is defined as a complex of sounds that cause a sensation of hearing discomfort, physical and psychological damage and, depending on the level, neurosis and hearing injuries such as tinnitus, temporary or permanent threshold change and communication interference (da Silva-Lopes, Zanlorenzi, Couto, & Minetti, 2004; Fiedler, de Lara-Santos, Corazza-Gatto, da Silva-Lopes, & da Silva-Oliveira, 2007; Otoghile, Onakoya, & Otoghile, 2018). Noise is one of the most common occupational hazards in sawmills and in the steel industry (Top, Adanur, & Öz, 2016), where manufacturing processes generate sound as an unwanted byproduct (Anjorin, Jemiluyi, & Akintayo, 2015).

On the other hand, light is a factor that has positive and negative effects on workers' performance; when the level is adequate, personal satisfaction increases, productivity improves, and fatigue and accidents are reduced (Lombardi, Pizzol, Vidaurre, Corletti, & Barbosa, 2011), but when light is insufficient or excessive it can cause errors or work accidents (Adu, Adu, Effah, Kwasi, & Antwi-Boasiako, 2015). It is therefore necessary to seek the ideal level of the amount of light available to perform a task that guarantees maximum performance and operator comfort (da Silva et al., 2004).

Based on the above, there is a need to study the physical factors of the environment in the work stations, to determine the risks to people and identify the critical areas that allow taking preventive measures and improving the work environment (Vanadziņš et al., 2010). Given the lack of information on the working environment conditions in sawmills in the region of El Salto, Durango, the present study focuses on analyzing the exposure of workers in their workstations to factors such as temperature, noise and lighting. This study assumes that these factors are at adequate levels that guarantee people's health, well-being, safety and productivity.

Materials and methods

Study area

The study was conducted in the town of El Salto, municipality of Pueblo Nuevo, Durango, Mexico. The analysis considered six sawmills for the production of long-dimension lumber, using vertical band saw towers from 127.0 to 254.0 mm (5 to 10") wide. The lumber is separated into six quality, thickness and nominal length categories, or marketed as mill-run (mix of grades) (Nájera-Luna et al., 2011).

Lumber production per shift between sawmills is variable: 16 000 board feet (37.73 m3) at the La Victoria sawmill; 12 000 board feet (28.30 m3) at the Quintana I and II sawmills; 10 000 board feet (23.58 m3) at El Diamante; 8 000 board feet (18.86 m3) at Pueblo Nuevo, and 6 000 board feet (14.5 m3) at García.

The field data considered only those jobs headed by machinery operators, since it is in these jobs that noise is generated and to which all workers are exposed to a greater or lesser extent. The list of jobs and the number of observations per sawmill are shown in Table 1.

Table 1. Jobs and field observations at six sawmills in El Salto, Pueblo Nuevo, Durango.

Job position Sawmill Number of observations
La Victoria El Diamante Aserradero García Quintana Pueblo Nuevo Quintana II
Sawyer 1 260
Forklift operator 1 260
Resaw operator 210
Swing saw operator 1 260
Edger operator 840

Methods

The data were obtained by directly measuring the variables at the workstations, using a digital environmental multimeter (MasTech® MS6300 5 in 1, China), which has sensors calibrated to record ambient temperature, relative air humidity, lighting, wind speed and noise level.

This study used the methodology of the following standards: a) NOM-011-STPS-2001 relating to health and safety conditions in workplaces where noise is generated (Secretaría del Trabajo y Previsión Social [STyPS], 2001); b) NOM-015-STPS-2001 regarding high or low thermal conditions-health and safety conditions (STyPS, 2002) and c) NOM-025-STPS-2008 concerning lighting conditions in workplaces (STyPS, 2008).

The field information was obtained by dividing the workday into three periods during the day; each period included a 10-minute observation time in which 10 readings were obtained for each variable. The first period was considered to be between 10:00 and 12:00 hours, representing the morning readings; from 12:00 to 14:00 hours, information was taken corresponding to the midday period and from 14:00 to 16:00 hours corresponding to the afternoon period.

Each workstation at each sawmill was monitored for seven workdays from April 19 to June 11, 2018, taking 21 environmental data collections per job. The variables were ambient temperature, relative air humidity, lighting, wind speed and noise level; in total, the database recorded 483 data gathering periods with 4 830 readings.

For noise level measurement, the multimeter sound level sensor was set to use the "A" weighting scale or slow response cycle with an integration threshold of 85 dB (A) and a measurement range of 85 to 115 dB(A).

Allowable noise exposure limit

The maximum allowable exposure time (MAET) equation stipulated by the Secretariat of Labor and Social Welfare in NOM-011-STPS-2001 (STyPS, 2001) was used. This standard determines the noise limit values at which workers, if exposed repeatedly, are not at risk of hearing loss induced by noise at the 90 dB(A) level in an 8-hour workday, using the following expression:

M A E T = 8 2 N E L - 90 3

Thermal comfort degree

The effective temperature (Et) equation that considers both wind speed and relative air humidity by Missenard in 1937 (Teodoreanu, 2016) was used; its validity, despite its age, makes it a classic bioclimatic index. This index is defined as the temperature in calm air that a healthy, sedentary subject, seated in the shade and dressed in work clothes, would perceive if the relative humidity were 100 % (Tejeda-Martínez, Luyando, & Jáuregui , 2011). The mathematical expression is as follows:

E t = 37 - 37 - a t 0.68 - 0.0014 r + 1 1.76 + 1.4 v 0.75 - 0.29 t a 1 - r 100

Statistical analysis

To test the normality hypothesis of the variables noise, effective temperature and lighting, the modified Shapiro-Wilks test was used, showing that these do not come from a population with a normal distribution; therefore, to detect statistical differences between jobs, sawmills and times of day, non-parametric analysis of variance and Kruskal-Wallis median range comparison tests (α = 0.05) were carried out. These analyses were performed with the InfoStat 2018 version program (Di Rienzo et al., 2018).

Results and discussion

Noise level

The information in Table 2 shows that the differences in the noise level between the jobs, sawmills and times of day were significant (P < 0.05). By workstation, the highest noise level was recorded in the head saw and resaw areas with an average of 100 dB(A), while lower levels of around 85 dB(A) were determined in the forklift operator's workstation. By sawmill, the noise generated by La Victoria's machinery is 6.9 % higher than that of the El Diamante sawmill, which had the lowest noise. Over the course of the day, results indicate that 2.4 % more noise is generated in the morning and midday than in the afternoon, due to the fact that humid air, which is greater in the morning, has a greater number of molecules that propagate sound (Ziemann, Barth, & Hehn, 2013).

Table 2. Noise conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.

Category Mean dB(A) TMPE (h) Median dB(A) Average of ranges* DF C H P
Jobs
Forklift operator 85.23 24.08 85.40 103.20 a 4 1 300.14 0.0001
Edger operator 89.71 8.55 90.75 198.03 b
Swing saw operator 91.69 5.41 91.60 230.11 b
Resaw operator 98.52 1.12 98.70 369.86 c
Sawyer 102.47 0.45 103.50 400.70 c
Sawmills
El Diamante 88.35 11.71 88.00 165.82 a 5 1 36.82 0.0001
Quintana 1 92.72 4.27 91.85 235.28 b
Pueblo Nuevo 92.51 4.48 94.20 243.00 bc
Quintana 2 93.61 3.47 91.85 252.92 bc
García 94.29 2.97 95.40 272.44 bc
La Victoria 94.92 2.57 93.20 280.72 c
Course of the day
Afternoon 91.39 5.80 90.40 216.77 a 2 1 8.39 0.0151
Midday 93.28 3.75 92.15 248.68 b
Morning 93.62 3.47 93.00 260.35 b
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.

The noise level varies between workstations and sawmills due to differences in the shape and size of the teeth of each type of band saw and circular saw (Krilek et al., 2016). According to Owoyemi, Falemara, and Owoyemi (2017), noise from operating saws ranges from 80 to 120 dB; moreover, even when idling, saws can produce noise levels up to 95 dB, which is in line with the 93 dB(A) of the present study, and the level rises to the extreme when the friction of the saw with the wood occurs.

The fact that the La Victoria sawmill generates the highest noise among the sawmills is probably because it has the largest head saw (10" wide = 254 mm). La Victoria has machinery in all of its workstations unlike the rest of the sawmills evaluated, where they generally lack a resaw and edger as is the case with García and Pueblo Nuevo. These sawmills use 5" wide (127 mm) band saws and, in spite of this, the noise level they generate is statistically comparable (P = 0.0001) with that of La Victoria, indicating greater control in the maintenance of the saws, machinery, and motors in the latter.

Electric motors contribute to raising the noise level in sawmills; in this regard, Nandi, Toliyat, and Li (2005) state that motors under normal operating conditions with balanced load and good alignment can cause fatigue failures that lead to increased vibration and noise, which justifies the recommended periodic maintenance.

In sawmills in southern Thailand, Thepaksorn et al. (2017) recorded high noise exposures that included maximum values of 94.4 dB (A), which are similar to those found in the present study in sawmills with the highest noise level.

Maximum allowable noise exposure time

According to NOM-011-STPS-2001, the noise level close to 90 dB(A) corresponds to a MAET of 8 h, while values around 100 dB(A) considerably reduce the MAET to less than one hour within an 8-hour workday (Table 2). Based on the foregoing, only the forklift and edger operators' workstations do not pose a risk from noise exposure. Among sawmills, the noise level recorded by El Diamante’s machinery also does not pose a risk to workers' health.

In some countries such as Brazil, Chile and Cuba, the average acceptable noise exposure standard in the workplace is 85 dB(A) over an 8-h period; however, this does not imply that there is a safe condition below this reference value, but it does indicate an acceptable level of risk to the worker's hearing health (Anjorin et al., 2015). In relation to the above and as a precautionary measure, it is recommended to implement protection protocols for workers in all the workstations and sawmills of the present study by virtue of exceeding 85 dB(A) of noise. According to Parsons (2000), in industrial settings, values of 90 dB(A) in 8-hour exposure periods have harmful effects on the health of workers in various physiological responses such as changes in heart rate, blood pressure and adrenaline production, as well as psychological impacts (mental health and emotional state).

Among the possible measures to implement in the sawmills under study, it is necessary to make the use of ear protectors mandatory, at least among the workers most exposed to high noise levels, such as machinery operators and their assistants. Also, recovery breaks could be made available depending on the intensity of the noise (Lombardi et al., 2011), as it has been proven that a plan of breaks or rotations every two hours is beneficial for the recovery of the worker (Tharmmaphornphilas & Norman, 2004). In this study, most sawmills implement three to four breaks during the workday, the first one from 8:30 to 9:00 in the morning for breakfast, the second one from 13:30 to 14:30 for lunch and also every three or four hours for changing saws, which in some way contributes to reducing the noise exposure time.

Petusk-Filipe et al. (2014) also note that it is possible to act against noise after it has been transmitted, by installing noise dissipaters around the machinery, which significantly reduces the intensity in workers' ears.

Thermal comfort zone

According to Table 3, the level of thermal comfort showed no significant difference between workstations (P = 0.1035), but it did between sawmills and during the course of the day (P = 0.0001). The average effective temperature was established at 20 °C with a difference of 0.98 °C between workstations; but between sawmills, differences of up to 3.06 °C and 2.38 °C were found between morning and afternoon.

Table 3. Effective temperature conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.

Category Mean (°C) Median (°C) Average of ranges* DF C H P
Job
Resaw operator 19.87 20.41 213.93 a 4 1 7.69 0.1035
Forklift operator 20.00 20.25 222.69 a
Edger operator 20.12 20.30 229.80 a
Swing saw operator 20.62 20.63 252.13 a
Sawyer 20.81 21.05 263.99 a
Sawmills
El Diamante 19.22 19.05 183.10 a 5 1 68.02 0.0001
Quintana 1 19.59 19.91 190.72 ab
García 20.13 20.88 229.63 bc
La Victoria 20.33 20.59 239.21 c
Quintana 2 21.22 21.59 292.45 d
Pueblo Nuevo 22.28 22.50 338.66 e
Course of the day
Morning 19.35 19.41 183.29 a 2 1 48.45 0.0001
Midday 20.30 20.58 240.70 b
Afternoon 21.52 21.73 302.39 c
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.

The thermal comfort zone is delimited by effective temperatures between 16 and 24 °C, relative air humidity from 40 to 60 % and air speed of 0.7 m·s-1; at less than 16.0 °C, discomfort due to cooling is generated, and at more than 24 °C, the heat is uncomfortable (da Silva-Lopes, Domingos, & Perrelli-Jarbas, 2006; Teodoreanu, 2016; Tutuş, Demir, Çiçekler, & Serin, 2018). Based on the above, the workstations in the sawmills under study are in adequate thermal comfort.

With respect to the stipulations of NOM-015-STPS-2001 for high thermal exposures of workers in 8-hour workdays, the 19 to 22 °C range recorded in this study does not pose a risk; that is, exposure to these temperatures in the workstations can be 100 %, since they are less than 25 to 30 °C for light, moderate and heavy work (STyPS, 2002).

In the present study, the fact that the thermal comfort of the work environment was adequate for workers is because the weather conditions during the information gathering period were not extreme (spring season). On the other hand, since the work is carried out in roofed spaces, but with large side openings, it is not possible to keep the temperature constant and regulated, since the air currents cause cooling inside the building. This could be accentuated in the winter time, when complaints of musculoskeletal discomfort or pain are likely to be intensified, due to working in cold environments; in addition, breathing cold air may lead to respiratory and cardiovascular problems, which in turn can decrease work performance (Mäkinen & Hassi, 2009).

Lighting level

During the study period, the recorded light intensity was high with a daily average of 12 870 lx. This is because the jobs in the evaluated sawmills are not in a closed environment and are located in places where they receive a lot of natural light. The results in Table 4 indicate that the lighting level showed significant differences between jobs and between sawmills (P = 0.0001), but not between daytime hours (P = 0.3304).

Forklift operators work in maximum light conditions because they work in open spaces with direct exposure to natural sunlight. For this reason, it is suggested that skin and eye protection be used during hours of greater illumination (Fiedler et al., 2007). Workstations under a roof recorded 89 to 96 % less light than the forklift operator area. The results showed that the incidence of light at the García sawmill is 38 % higher than El Diamante’s, probably because the latter’s roof is lower than other structures; even so, the lighting level is above the recommended international labor standards for sawmills, which vary from 500 to 2 000 lx (Alves et al., 2002; Lombardi et al., 2011).

Lack of lighting does not pose a risk to workers in these sawmills, and the use of eye protection is even recommended to attenuate the excessive amount of light to avoid problems of visual fatigue, incidence of errors, decrease in work performance and accidents due to the direct effect of glare, brightness and flashes. Another technical solution is the installation of shade mesh in strategic sawmill areas to minimize the incidence of excessive light in the workstations that have this problem.

Table 4. Lighting conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.

Category Mean (lx) Median (lx) Average of ranges* DF C H P
Job
Edger operator 1 817.50 1 038.50 162.43 a 4 1 260.29 0.0001
Swing saw operator 2 828.70 895.00 169.58 a
Sawyer 2 466.60 992.00 196.82 ab
Resaw operator 4 606.70 1 459.00 248.93 b
Forklift operator 42 063.00 45 750.00 411.50 c
Sawmills
El Diamante 10 758.00 396.50 160.29 a 5 1 57.80 0.0001
La Victoria 11 360.00 873.00 214.03 b
Pueblo Nuevo 15 207.00 1 366.00 246.60 bc
Quintana 2 12 895.00 1 436.00 270.59 cd
Quintana 1 11 562.00 1 191.50 271.60 cd
García 17 582.00 2 840.00 315.39 d
Course of the day
Midday 13 119.00 1 080.00 234.23 2 1 2.21 0.3304
Morning 12 637.00 1 205.00 236.53
Afternoon 12 855.00 1 880.50 255.36
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.

Visual performance is a concept related to the combination of the eye's effectiveness in receiving and conditioning itself to light, and to the interpretation of what a person sees. This is important because it can influence the interpretation of what workers see, and in practice an adequate level of lighting is required to promote the desired visual performance for particular job tasks (Parsons, 2000).

Finally, in this study, among the visual categories per position and work area of NOM-025-STPS-2008, lighting is classified as highly accurate to a high degree of specialization in distinguishing details, as it exceeds the 2 000 lx reference standard. This indicates that the amount of light is greater than that required at the evaluated sawmills’ work stations.

Conclusions

Of the three environmental variables evaluated in the sawmills, noise poses the greatest risk to workers' health, as they are exposed to levels exceeding 90 dB(A) throughout the workday. The lighting level in the workstations was over the 2 000 lx recommended by international standards; however, the level is not constant during the day, so the risk to the worker decreases. During the assessment period, the only variable at appropriate levels was thermal comfort, as the 20 °C average guarantees workers 100 % exposure to this temperature during the working day. Based on the above, the health and safety of workers can be guaranteed by the mandatory use of ear and eye protectors during the workday.

Acknowledgments

  • The authors would like to thank the Durango State Science and Technology Council (COCYTED) for funding the research project "Evaluation of environmental conditions and risks in forestry industry jobs in El Salto, Durango" from which this paper originated.

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Tables:

Table 1. Jobs and field observations at six sawmills in El Salto, Pueblo Nuevo, Durango.
Job position Sawmill Number of observations
La Victoria El Diamante Aserradero García Quintana Pueblo Nuevo Quintana II
Sawyer 1 260
Forklift operator 1 260
Resaw operator 210
Swing saw operator 1 260
Edger operator 840
Table 2. Noise conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.
Category Mean dB(A) TMPE (h) Median dB(A) Average of ranges* DF C H P
Jobs
Forklift operator 85.23 24.08 85.40 103.20 a 4 1 300.14 0.0001
Edger operator 89.71 8.55 90.75 198.03 b
Swing saw operator 91.69 5.41 91.60 230.11 b
Resaw operator 98.52 1.12 98.70 369.86 c
Sawyer 102.47 0.45 103.50 400.70 c
Sawmills
El Diamante 88.35 11.71 88.00 165.82 a 5 1 36.82 0.0001
Quintana 1 92.72 4.27 91.85 235.28 b
Pueblo Nuevo 92.51 4.48 94.20 243.00 bc
Quintana 2 93.61 3.47 91.85 252.92 bc
García 94.29 2.97 95.40 272.44 bc
La Victoria 94.92 2.57 93.20 280.72 c
Course of the day
Afternoon 91.39 5.80 90.40 216.77 a 2 1 8.39 0.0151
Midday 93.28 3.75 92.15 248.68 b
Morning 93.62 3.47 93.00 260.35 b
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.
Table 3. Effective temperature conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.
Category Mean (°C) Median (°C) Average of ranges* DF C H P
Job
Resaw operator 19.87 20.41 213.93 a 4 1 7.69 0.1035
Forklift operator 20.00 20.25 222.69 a
Edger operator 20.12 20.30 229.80 a
Swing saw operator 20.62 20.63 252.13 a
Sawyer 20.81 21.05 263.99 a
Sawmills
El Diamante 19.22 19.05 183.10 a 5 1 68.02 0.0001
Quintana 1 19.59 19.91 190.72 ab
García 20.13 20.88 229.63 bc
La Victoria 20.33 20.59 239.21 c
Quintana 2 21.22 21.59 292.45 d
Pueblo Nuevo 22.28 22.50 338.66 e
Course of the day
Morning 19.35 19.41 183.29 a 2 1 48.45 0.0001
Midday 20.30 20.58 240.70 b
Afternoon 21.52 21.73 302.39 c
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.
Table 4. Lighting conditions in the evaluation categories in six sawmills in El Salto, Pueblo Nuevo, Durango.
Category Mean (lx) Median (lx) Average of ranges* DF C H P
Job
Edger operator 1 817.50 1 038.50 162.43 a 4 1 260.29 0.0001
Swing saw operator 2 828.70 895.00 169.58 a
Sawyer 2 466.60 992.00 196.82 ab
Resaw operator 4 606.70 1 459.00 248.93 b
Forklift operator 42 063.00 45 750.00 411.50 c
Sawmills
El Diamante 10 758.00 396.50 160.29 a 5 1 57.80 0.0001
La Victoria 11 360.00 873.00 214.03 b
Pueblo Nuevo 15 207.00 1 366.00 246.60 bc
Quintana 2 12 895.00 1 436.00 270.59 cd
Quintana 1 11 562.00 1 191.50 271.60 cd
García 17 582.00 2 840.00 315.39 d
Course of the day
Midday 13 119.00 1 080.00 234.23 2 1 2.21 0.3304
Morning 12 637.00 1 205.00 236.53
Afternoon 12 855.00 1 880.50 255.36
*Ranges with the same letter in each category are not significantly different according to the Kruskal Wallis test (α = 0.05). DF: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties.