Forest work is heavy work — Ergonomic measurement techniques — tools for work design

Short Version of the presentation by Prof. Dr. Bombosch

Forest work is physically strenuous. A portable measurement system has been developed to estimate these burdens. Additional to physical stress forest workers are interfered with carbon monoxide emissions from chainsaws. However, this "invisible" threat had to be made visible - with a simple trick. Forest workers are prone to accidents. They often tumble and stumble because of low vision in their working environment with strong contrasts. Special glasses can help.

Dimension of forest work

Figure 1: Assessment of domestic raw timber use in 2006 (Click to enlarge)

In the Federal Republic of Germany , the number of professional forest workers has declined in recent decades, from about 30 000 to 9600. This decline was largely offset by a high mechanization and on the other hand by shifting capacity to entrepreneurs with rather less professional staff. It should be noted that today a set of 100 000 and more unprofessional people do temporarily hard forestwork in the field of firewood production.

Mobile ergonomic measuring unit for stress-strain analysis of piecework

Figure 2: Scheme of the mobile measurement unit (Click to enlarge)

As part of the maintenance of the piecerate system in the forest work in West Germany, the parties for the wage system agreed that a mobile ergonomic measuring unit should be purchased in order to realize a stress - strain analysis at piecework. Based on the results and conclusions the decision about the continuation of the wage system should be taken.

The unit was developed by the author and Med-NATIC GmbH and Rimkus Elektrotechnik at the Forest Research Station in Freiburg.

The most important parameter in addition to heart rate, activity, noise, and skin temperature (minimum recording/averaging rasters are 5 sec), is the timestudy , which allows the designer to map the stresses, the stress moments and evaluate the situation.

Heart rate over working day

Daily histograms of heart rate about a working day showed that among professional forest workers distributions with two peaks occur that are usually only found in high-performance sports. Under normal conditions the cardiovascular system is well secured and programmed to the professional forest workers.

Figure 3: Heart rate of a 32-year-old forest worker during a working day (Click to enlarge)

Result of the measurements of heart rates

Exhaust pollution by power saws

Figure 4: Measurement unit exhaust pollution (Click to enlarge)

The Regulations of Hazardous Substances published 26 th August 1986 charges the employer to measure and assess the threats. Furthermore he has to make a documentation of the results and give information to interested parties.

Massive criticism of the chain saw manufacturers and downplaying the problems by the employer initially accompanied these activities. Only with a competent network consisting out of the Institute of Occupational Health and Safety in Göttingen and the IAS in Karlsruhe could address this situation.

Since 1990 a mobile measurement method for carbon monoxide and blood gas analysis were developed at the HAWK in Göttingen. The sensors from Bayer Diagnostics , Dositox and blood gas detector Radiometer were modified for the use in the forests.

In the exhaust gases of 2-stroke engines are numerous toxic substances. Carbon monoxide by 70% represents the majority. After inhalation, the carbon monoxide binds to the blood pigment hemoglobin, which ensures the vital oxygen around the body. Since the binding of the carbon monoxide is 200x stronger than that of oxygen, the oxygen supply is rapidly deteriorated. The risk of poisoning by the odorless and colorless carbon monoxide gas increases as the concentration in the air and the duration of inhalation (exposure).

Exhaust gases in the forest

Visualization of exhaust gases

Figure 5: test procedure for the Visualization of exhaust gases (Click to enlarge)

2005 the HAWK was commissioned by the State Accident Insurance in Hanover to make a training film on the subject of exhaust pollution.

The visualization of the exhaust gases with sunflower oil in the fuel caused that the warnings were taken seriously in the film.

The results of the studies are still relevant today defused by the design of the workplaces of chainsaw operators and the introduction of benzol free alkylate fuels.

Co-immissions in the heavy timber harvest

Figure 6: Exhaust gases in the harvest of strong timber (Click to enlarge)

Immission of carbon monoxide in the processing of two strong spruces. Tree A (3.46 fm), Tree B (3.6 fm)

Emissions by chainsaws

Figure 7: Comparison of Chainsaws (Click to enlarge)

Light as load parameter

Risks caused by visual impairments

Figure 8: Die menschliche Netzhaus (Retina) (Click to enlarge)

Stumble comes first as an accident cause in the forest work. As part of a study to investigate the causes of accidents in the timber harvest (Günter Salow), it turned out that the visor visibility severely hampered by the forest workers helmet for more than 50% of forest workers and directly or indirectly affect the reactions in case of danger.

The human retina is composed of rods and cones, the cones, the photoreceptors for seeing in the light of higher intensity (by day) and for color vision are. The rods allow vision in low light intensity (at night), with no color information is perceived (Black - White - vision). In addition, chopsticks are more likely in the peripheral areas of the retina, so that the images are perceived rather blurred at night. The cones contribute to the perception of color blue, green and yellow sensitive photopigments and are in the center of the retina.

Figure 9: susceptibility of the human eye depending on light conditions (Click to enlarge)

The photopigments are not only unequally distributed over the cones (64% of suppositories containing yellow, 32% green and about 2% of suppositories containing blue pigments), also the cones themselves are unevenly distributed on the retina. In the center of the retina is the green area, surrounded by the yellow area, which is in turn surrounded by the blue area.

Lighting conditions in the forest

Figure 10: Lighting conditions in the forest (Click to enlarge)

Contrast ratios of 1:10 000 have been measured in forest work.

Solution for good vision in the forest

Only through the personal experience of this effect, it is possible to convince the better sight for the forest workers. The combination of safety glasses and the effect as described is remarkable.

Figure 11: The right eyewear allows good vision in the forest (Click to enlarge)

The highest sensitivity of the eye in humans is 555 nm which is optimally served by wearing yellow glasses in predominantly green forests. Gray mirror glasses are able at high light intensity to switch off lightcones and to improve the contrast (3- D vision) by "switching on" the sticks.