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Lifting
The spine is a sensitive mechanism. Over
time, improper lifting can cause irreversible damage that can lead to permanent injury.
Issue -
High load weights, repetitive lifting of heavy loads and improper workstation design
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 At
some facilities, workers were required to lift 100 pound sacks of potatoes. This
weight is well above the threshold of what one person can lift safely. There is
concern that handling these weights is putting these workers at a significant risk of
developing cumulative
trauma injuries as well as acute
back injuries. The risk to the worker is increased due to the unstable nature of heavy
loads packaged in sacks. The workstation design requires workers to lift loads above
their shoulders and below their knees which also increases the risk of both cumulative and
acute injuries. |
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 Workers
were observed making repetitive lifts of 50 pound boxes at a workstation that exposes them
to risk of injury. Workers were observed lifting 50 pound boxes of potatoes from roller belt lines
only 16 to 19 inches from the ground. The lifting height of the box is below the knee
height of most workers. These boxes were lifted and placed on pallets that were stacked to
heights above the workers' heads. This task puts workers at risk of back and shoulder
injury.
The NIOSH lifting equation is
a tool used to assess a lifting task and given the conditions,
determine a Recommended Weight Limit (RWL) for the task.
The RWL is calculated using the equation which starts with a lifting
constant of 51 pounds and takes into account characteristics of
the load and the lifting conditions such as, horizontal (H)
and vertical (V) location
of load, the distance
the load will be moved (D), asymmetry
(A), frequency of the
lift (F), hand coupling
for the load (C), and the length
of time during which the lifting will take place.
The starting point of 51 pounds is discounted by these variables
resulting in a lower weight suited for the conditions. Thus,
the recommended weight that should not be exceeded assuming ideal
lifting conditions and techniques is 51 pounds. Ideal conditions
are not realistic when calculating a recommended weight limit
for most tasks, thus 51 pounds will invariably be reduced to a
lower weight.
The following examples illustrate what the recommended weight limit for the conditions
under which the task is being performed.
Situation 1
  The
worker is lifting a bag from a starting vertical height of 34 inches and placing the box
at a vertical destination above the shoulders, approximately 65 inches high. The box is
held reasonably close to his body (use best case of 10 inches). The shape of the box is
good, but there are no cut-outs or handles for the hands, so the hand-to-container
coupling classification is fair. The worker is performing the lift with little asymmetry.
The worker handles approximately 4 boxes per minute and is lifting these 50 pound boxes
throughout his eight hour shift. The variables used in the
NIOSH equation, in this case, H=10 inches, V=34 inches, D=29 inches, A= 1, coupling
classification is fair, frequency is 4 lifts/minutes and the duration is 8 hours.
With these conditions, the recommended weight limit (RWL) is 20 pounds at the origin
and 15 pounds at the destination. The lifting index (LI) is 2.6 at the origin and 3.4 at the
destination. An LI value of greater than 1 indicates the task is hazardous for some
portion of the population and a value greater than 3 indicates the job is likely to be a
problem for much of the population.
Situation 2
 This worker is lifting 50 pound boxes coming down a line that is
about mid-thigh height, approximately 28 inches from the ground. The worker holds the box
reasonably close to his body (10 inches) when he retrieves it, however, when he places it
at the destination, the box is away from his body (24 inches) as he places it behind
another box on the pallet. The vertical height is approximately 30 inches at the boxes
destination. Hand coupling is fair and asymmetry is minimal. Specifically in this case, H=10 inches at origin (24 inches at destination), V=28 inches, D=2 inches,
A= 1, coupling classification is fair, frequency is 3 lifts/minutes and the duration is 8
hours.
Using this situation, the RWL is 28 pounds at the origin and only 12 pounds at the
destination, giving the task a lifting index of 1.8 at the origin and 4.3 at the
destination.
Note that for both situations, as the pallets are loaded with boxes of potatoes, the
destination height will increase and placement of the box will vary, however, the origin
remains constant. Thus, even though these workers are doing this job over the course
of their shift, the numbers will vary slightly. Although these examples may
demonstrate extreme situations (one worker placing the box about head height, and the
other reaching far away from his body), this task is still of concern. For instance,
the extreme situation in both cases is at the destination. The box origin for the
lift is not too extreme and yet the RWL is 20 pounds for situation 1 and 28 for situation
2. This is due to the repetition involved and the task being carried out over an
eight hour shift.
The Army's MIL-STD-1472, Human Engineering
Design Criteria for Military Systems, Equipment and Facilities,
addresses lifting, and it was examined to confirm recommended lifting limits. Using
just a single lift evaluation (not repetitive, as is the case with these workers) for a
lift performed by the male only population, a lift from the floor to a surface not
greater than 5 feet above the floor is 56 pounds (not much more than these potato boxes).
However, when you take into consideration the frequency of the lifts (if the worker is
lifting 4 per minute), this number is reduced to 37 pounds. At three lifts
per minute, the maximum load under these conditions is 42 pounds! |
Issue
- Back Belts
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At some facilities, workers were observed wearing back
belts. In those cases, it appeared that the belts had been distributed to workers
across the board. Currently, evidence on the effectiveness of back belts is
inconclusive and is currently being researched by NIOSH. Providing back belts to all
employees is not recommended; they should be distributed only on a case by case
basis. It is more effective to assess the lifting task to ensure that it can be
accomplished without exceeding the capabilities of the worker and make appropriate
changes. Wearing a back belt is a band aid approach where the real problem may not
be addressed. When workers are given back belts, they may attempt to lift more
weight than they would without it. NIOSH recommends that employers and workers
minimize their risk of back injury by developing and implementing a comprehensive
ergonomics program. See NIOSH's SELECTED TOPICS Ergonomics and Musculoskeletal
Disorders to learn more on the current back belts information available from NIOSH,
including Back Belts - FACTS: June 1997,
Back Belts -- Do They Prevent Injury?
and Workplace Use of Back Belts Review
and Recommendations. |
Recommendations
Both the NIOSH lifting equation and the Army standards indicate that these lifts
are outside the recommended safe lifting guidelines. At this point, administrative
and engineering controls are appropriate and need to be implemented to reduce the risk of
injury to the workers. The following recommendations are made:
 First, the potato
harvesting manufacturers should discontinue packing potatoes in 100 pound sacks because it
places their workers at risk. Then, the distributors should be informed that it is not to
their benefit to request 100 pound bags because it places their workers at an increased
risk of injury when handling these heavy loads. Lifting a 100 pound object is a two person
lift. As a repetitive task, this is not suitable for workers to be handling and would only
be acceptable if transported by machines. It is strongly recommended that the potatoes
packaged in 100 pound sacks no longer be used. Sacks should be avoided due to the
potential for the load to shift while handling. If there is a concern that
distributors want 100 pound bags then it is important to educate the distributors to the
fact that their workers are at risk too.
Second, the optimal vertical lifting height for handling heavy materials is between the
shoulder and knees of the worker. The figure at the right (Pheasant, 1996) illustrates the
preferred area for handling materials and categorizes zones which are not preferred. Note
that the worst scenario is above shoulders, below knees and anything more than a forearm
length distance from the body. Workers should avoid bending low or lifting high to reduce
the risk of injury. Boxes rolling off the line should be channeled to a height
approximately 33 inches from the ground. This would put the boxes about hip height for
most workers. Boxes should be piled on pallets no higher than shoulder height,
approximately 50 inches. Additional benefit would be gained from using a spring loaded
lift for the pallets to keep the workers from bending over or stooping to ground
level. These changes to the workstations where 50 pound boxes are handled would help
to reduce the risk of injury to workers. However, a long term goal should be to set the
standard of the weight of packaging potatoes to a lesser amount. There is no basis for why
the boxes need to be 50 pounds. And rather than arbitrarily select a new number, a
thorough analysis of the task would need to be conducted to determine an appropriate
weight for these boxes.
Finally, a Materials Handling Checklist may be
useful to employers and employees to help reduce the risk of developing back injuries in
the workplace.
Note: an Applications
Manual for using the NIOSH Lifting Equation is available online and should be
consulted before using the formulas for designing tasks.
NIOSH has a guide, Elements of Ergonomics
Programs that provides basic information useful
for employers, workers, and others in designing effective programs to prevent work-related
musculoskeletal disorders (WMSDs).
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