How to use the revised NIOSH Equation

Do you need to know how to use the revised NIOSH Equation to reduce manual lifting and lowering stress?

The NIOSH lifting equation tool was developed to assess and evaluate the safety limits two-handed lifting tasks to assist employers in reducing the risk of lifting-related injuries. Designed by the National Institute for Occupational Safety and Health (NIOSH), the tool assists occupational health and safety professionals to identify solutions for reducing the physical stress associated with manual lifting and lowering.

According to Worksafe, it is not applicable to manual handling activities such as pushing, pulling, carrying or holding.

The NIOSH Equation helps to answer two common questions relating to lifting and lowering:

1/ What is considered a safe weight to lift in a particular situation or circumstance?

2/ What can be done to make this lifting safe.

How do I use the NIOSH Lifting Equation?

The calculation requires first the collection of some vital information about the task, including:

• what is the weight of the object being lifted?
• how often is the lifting done?
• for how long is the lifting done?
• what is the height of the hands at the start and end of the lift?
• how far away are the hands from the body at the start and the end of the lift?
• how good a grip can the employee get on the object?
• what is the degree of twisting to the body?

Simply, the NIOSH lifting equation measures whether the weight of the object is too heavy for the task. The goal is to design all lifting jobs to accomplish a Lifting Index (L1) of less than 1.0.

The NIOSH Equation is used in the context of the Victorian Manual Handling Regulations and Codes of Practise.

You can now download the NIOSH Lift Calculator from the App Store.

Alternatively, contact us to find out more about how you can apply the NIOSH Equation to create a safer workspace for your employees.

The NIOHS Equation and its function 

For the more dedicated OH&S professionals, please find below detailed definitions and calculation descriptions.

1.1 Recommended Weight Limit (RWL)

Recommended Weight Limit is the principal product of the revised NIOSH lifting equation.

The Recommended Weight Limit is the weight of the load that nearly all healthy workers could perform in a specific set of task conditions over a substantial period of time (eg. up to 8 hours) without an increased risk of developing lifting-related low back pain.

The revised lifting equation for calculating the RWL is based on a multiplicative model that provides a weighting for each of six task variables. The weightings are expressed as coefficients that serve to decrease the load constant.

The load constant represents the maximum recommended load weight to be lifted under ideal conditions.

The RWL is defined by the following equation:

RWL = LC x HM x VM x DM x AM x FM x CM

Where :

LC Load Constant = 23 kg.

HM Horizontal Multiplier

VM Vertical Multiplier

DM Distance Multiplier

AM Asymmetric Multiplier

FM Frequency Multiplier

CM Coupling Multiplier The term multipliers refers to the reduction coefficients that serve to decrease the load constant. 1.2 Lifting Index (LI)

The Lifting Index (LI) is a term that provides a relative estimate of the level of physical stress associated with a particular manual lifting task.

The Lifting Index is defined by the following equation :

LI = Load Weight (L) ______ Recommended Weight Limit (RWL)

Load Weight (L) = weight of the object lifted.

Lifting tasks with a Lifting Index greater than 1.0 pose an increased risk for lifting-related low back pain.

If the magnitude of the LI increases: (1) the level of the risk for the worker performing the job would be increased; and (2) a greater percentage of the workforce is likely to be at risk for developing lifting- related low back pain.

It is believed that nearly all workers will be at an increased risk of a work-related injury when performing highly stressful lifting tasks (ie. lifting tasks that would exceed a LI of 3.0).

Therefore, the goal should be to design all lifting jobs to achieve a LI of 1.0 or less.

2 TASK DATA or VARIABLES

The term task data or task variables refers to the task descriptors (ie. Horizontal location, Vertical location, Distance of travel, Asymmetric angle, Frequency rates and Coupling) that are measurable.

2.1 Horizontal Location (H) Horizontal Location (H) is measured from the mid-point of the line joining the inner ankle bones to a point projected on the floor directly below the mid-point of the hand grasps (ie. load center), as defined by the large middle knuckle of the hand.

Horizontal Location (H) should be measured. In those situations where the H value cannot be measured, then H may be approximated from the following equations :

1) for V = >25 cm H = 20 cm + W/2

2) for V <25 cm H = 25 cm + W/2 Where : W is the width of the container in the sagittal plane and V is the vertical location of the hands from the floor.

2.2 Vertical Location (V) Vertical Location (V) is defined as the vertical location of the hands above the floor at origin of lift.

V is measured vertically from the floor to the mid-point between the hand grasps, as defined by the large middle knuckle.

2.3 Vertical Travel Distance (D) The Vertical Travel Distance (D) is defined as the vertical travel distance of the hands between the origin and destination of the lift.

D = V at destination – V at origin

The vertical distance (D) is assumed to be at least 25 cm, and not greater than 175 cm. If the vertical travel distance is less than 25 cm, then D should be set to the minimum distance of 25 cm.

2.4 Asymmetric Angle (A) Asymmetry refers to a lift that begins or ends outside the mid-sagittal plane as shown in Figure 2.

The asymmetric angle (A), which is depicted graphically in Figure 2, is operationally defined as the angle between the asymmetry line and the mid-sagittal line.

The asymmetry line is defined as the horizontal line that joins the mid-point between the inner ankle bones and the point projected on the floor directly below the mid-point of the hand grasps, as defined by the large middle knuckle.

The sagittal line is defined as the line passing through the mid-point between the inner ankle bones and lying in the mid-sagittal plane, as defined by the neutral body position (ie. hands directly in front of the body, with no twisting at the legs, torso or shoulders).

The asymmetric angle is not defined by foot position or the angle of torso twist, but by the location of the load relative to the worker’s mid-sagittal plane.

2.5 Lifting Frequency (F) Average number of lifts per minute, as measured over a 15 minute period..

* If the worker does not lift continuously for 15 minutes, we need to use a special procedure to determine the appropriate lifting frequency. This procedure is presented below:

1. Compute the total number of lifts performed for the 15 minute period Lifting rates x work time Lifting rate: the number of actual lifts per minute

2. Divide the total number of lifts by 15: Total number of lifts

15

Following are a few examples of how to calculate lifting frequency for the 15-minute period:

Example 1: The work pattern consists of:

8 minutes of lifting ==> 15 minute period sampling 7 minutes of non-lifting work 8 minutes of lifting 7 minutes of non-lifting work 8 minutes of lifting…………. and goes on like this for 2 hours. The lifting rate we count is 10 lifts per minute.

1) The total number of lists performed for the 15 minute period sampling: 10 lifts x 8 = 80 lifts

2) Divide the total number of lifts by 15: 80 = 5.33 lifts per minute 15

Example 2: The work pattern consists of intermittent, six-minute lifting sessions separated by three-minute light work periods. The lifting rate we count is 3 lifts per minute.

15 minute sampling period consists of:

6 minutes of lifting 3 minutes of non-lifting 6 minutes of lifting

1) The total number of lifts performed for the 15 minute period: 3 lifts x 12 (or 3 lifts x 6 minute session x 2 sessions) = 36 lifts

2) Divide the total number of lifts by 15: 36 = 2.4 lifts/min 15

Lifting above the maximum frequency results in RWL of 0.0. (Except for the special case of discontinuous lifting discussed above, where the maximum frequency is 15 lifts/minute.)

2.6 Coupling (C) Coupling refers to the relationship between the hands and the object.

2.6.1. Coupling Classification Coupling must be classified as good, fair, or poor dependent on the nature and dimensions of the object and gripping method.

GOOD Containers of optimal design (boxes, crates, etc.) with handles or hand-hold cut-outs of optimal design (40 cm x 30 cm).

Loose parts or irregular objects (castings, stock, and supply materials): comfortable grip in which the hand can be easily wrapped around the object FAIR Containers or/objects of optimal design with handles or hand-hold cut-outs less than optimal design

Containers of optimal design with no handles or hand-hold cut-outs. Loose parts or irregular objects: coupling classified as FAIR if the hand can flex about 90 degrees when gripping. POOR Containers of less than optimal design or loose parts or irregular objects : bulky, hard to handle, having sharp edges.

Non-rigid bags, sagging bags

The effectiveness of the coupling may vary with the distance of the object from the ground, so that a good coupling could become a poor coupling during a single lift. The entire range of the lift should be considered when classifying hand-to-object couplings, with classification based on overall effectiveness.

2.6.2. Optimal Container Design An optimal container/object design has 40 cm frontal length, 30 cm height, and a smooth non-slip surface.

A container is considered less than optimal if it has a frontal length less than 40 cm, height less than 30 cm, rough or slippery surface, sharp edges, asymmetric center of mass, unstable contents, or requires the use of gloves.

2.6.3. Bulky Object A loose object is considered bulky if the load cannot easily be balanced between the hand-grasps.

2.6.4. Optimal Handle / Hand-hold Cut-out Design An optimal handle design has 1.9 to 3.8 cm diameter, 11.5 cm length, 5 cm clearance, cylindrical shape, and a smooth, non-slip surface.

An optimal hand-hold cut-out has the following approximate characteristics: less than or equal to 3.8 cm height, 11.5 cm length, semi-oval shape, less than or equal to 5 cm clearance, smooth non-slip surface, and less than or equal to 0.60 cm container thickness (e. g. double thickness cardboard).

2.7 Significant Control Significant control is defined as a condition requiring precision placement of the load at the destination of the lift. This is usually the case when the worker has to : (1) re-grasp the load near the destination of the lift, or (2) momentarily hold the object at the destination, or (3) carefully position or guide the load at the destination.

3. MULTIPLIERS

The multiplier values can be determined from Tables (Appendix 1)

3.1 Horizontal Multiplier The HM value can be determined from Table 1.

If H is less than or equal to 25 cm, then the multiplier is 1.0. HM decreases with an increase in H value. The multiplier for H is reduced to 0.4 when H is 63 cm. If H is greater than 63 cm, then HM = 0.

3.2 Vertical Multiplier The VM value can be determined from Table 2.

When V is at 75 cm, the vertical multiplier (VM) is 1.0. The value of VM decreases linearly with an increase or decrease in height from this position. At floor level, VM is 0.78, and at 175 cm height VM is 0.7. If V is greater than 175 cm, then VM = 0.

3.3 Distance Multiplier The DM value can be determined from Table 3.

Id D is less than 25 cm, D is assumed to be 25 cm and DM is 1.0. DM is 0.85 when D = 175 cm. The Distance Multiplier, therefore, decreases gradually with an increase in travel distance. Thus, DM ranges from 1.0 to 0.85 as the D varies from 0 cm to 175 cm.

3.4 Asymmetric Multiplier The AM value can be determined from Table 4.

The asymmetric angle is limited to the range from 0 degree to 135 degrees. If A is greater than 135 degrees, then AM = 0, and the load is zero.

3.5 Frequency Multiplier The FM value is determined from Table 5.

For lifting tasks with a frequency less than .2 lifts /minute, set the frequency equal to .2 lifts/minute.

The frequency multiplier value depends upon: (a) the average number of lifts per minute (F), (b) the vertical location (V) of the hands at the origin, and (c) the duration of continuous lifting.

3.6 Lifting Duration Lifting duration is classified into three categories – short duration, moderate duration and long duration. These categories are based on the pattern of continuous (uninterrupted) work-time and recovery-time (ie. light work)

Short duration defines lifting tasks that have a work duration of one hour or less, followed by a recovery time equal to 1.2 times the work time.

Examples:

To be classified as short-duration, a 45-minute lifting job must be followed by at least a 54-minute recovery period prior to beginning a subsequent lifting session. If the required recovery time is not met for a job of one hour or less, and a subsequent lifting session is required, then the total lifting time must be combined to correctly determine the duration category. If the recovery period does not meet the time requirement, it is disregarded for purposes of determining the appropriate duration category.

A worker lifts continuously for 30 minutes, then performs a light work task for 10 minutes, and then lifts for an additional 45-minute period. In this case, the recovery time between lifting sessions (10 minutes) is less than 1.2 times the initial 30 minute work-time (36 minutes). Thus, the two work times (30 minutes and 45 minutes) must be added together to determine the duration. On the other hand, if the recovery period between lifting sessions was increased to 36 minutes, then the short-duration category would apply.

Moderate duration defines lifting tasks that have a duration of more than one hour, but not more than two hours, followed by a recovery period of at least 0.3 times the work time.

If a worker lifts continuously for 2 hours, then a recovery period os at least 36 minutes would be required before commencing a subsequent lifting session. If the recovery time requirement is not met, and a subsequent lifting session is required, then the total work time must be added together. If the total work time exceeds 2 hours, then the job must be classified as a long-duration lifting task.

Long duration defines lifting tasks that have a duration of between two and eight hours, with standard industrial rest allowances (eg. morning, lunch and afternoon rest breaks).

NOTE : No weight limits are provided for more than eight hours of work.

3.7 Coupling Multiplier

Based on the coupling classification and vertical location of the lift, the Coupling Multiplier (CM) is determined from Table 7.

4. DESIGN/REDESIGN USING THE RECOMMENDED WEIGHT LIMIT AND LIFTING INDEX

The Recommended Weight Limit and Lifting Index can be used to guide ergonomic design:

1) The individual multipliers can be used to identify specific job-related problems. The relative magnitude of each multiplier indicates the relative contribution of each factor of the task (horizontal, vertical, frequency, etc)

2) The Recommended Weight Limit can be used to guide the redesign of existing manual lifting jobs or to design new manual lifting jobs. For example, if the task variables are fixed, then the maximum weight of the load could be altered so as not to exceed the Recommended Weight Limit. If the weight is fixed, then the task variables could be optimised so as not to exceed the Recommended Weight Limit.

3) The Lifting Index can be used to estimate the relative magnitude of physical stress for a job. The greater the Lifting Index, the smaller the fraction of workers capable of safely sustaining the level of activity.

5. LIMITATIONS

The equation does not apply in the following situations as it could either under – or over-estimate the extent of physical stress associated with a particular lifting task : * Lifting/lowering with one hand * Lifting/lowering for over 8 hours * Lifting/lowering while seated or kneeling * Lifting/lowering in a restricted work space * Lifting/lowering unstable objects. An unstable object or load is an object in which the location to the center of mass varies significantly during the lifting activity, such as some containers of liquid or incompletely filled bags, etc. * Lifting/lowering while carrying, pushing or pulling. The equation still applies if there is a small amount of holding and carrying, but carrying should be limited to one or two steps and holding should not exceed a few seconds. * Lifting/lowering with wheelbarrows or shovels * Lifting/lowering with high speed motion (faster than about 30 inches/second) * Lifting/lowering with unreasonable foot/floor coupling (<0.4 coefficient of friction between the sole of the floor) that may increase the risk of a slip or fall. * Lifting/lowering in an unfavourable environment (ie. temperature significantly outside 19-26 degrees C range; relative humidity outside 35-50% range)

6. SINGLE TASK AND MULTI TASK ASSESSMENTS

6.1 Single task job assessment A single-task manual lifting job is defined as a lifting job in which: * the task variables do not significantly vary from task to task, * or only one task is of interest (eg. worst case analysis). This may be the case if the effects of the other tasks on strength, localised muscle fatigue, or whole-body fatigue do not differ significantly from the worst case task.

6.2 Multi task job assessment On the other hand, multi-task manual lifting jobs, which are defined as jobs in which there are significant differences in task variables between tasks, are more difficult to analyse because each task must be analysed separately. Therefore, a specialised procedure is used to analyse multi-task manual lifting jobs.

6.3 Multi-Task Procedure

After determining multipliers, the following steps will be followed:

1. Compute the Frequency-Independent Recommended Weight Limit (FIRWL): FIRWL = 23 x HM x VM x DM x AM x CM

2. Compute Single-Task Recommended Weight Limit (STRWL) for each task: STRWL = FIRWL x FM

3. Compute the FILI for each task:

FILI = Object Weight FIRWL

4. Compute the Single-Task Lifting Index (STLI) for each task:

STLI = Object weight STRWL

5. Renumber the tasks in order of decreasing physical stress, beginning with the task with the greatest Single Task Lifting Index down to the task with the smallest Single Task Lifting Index.

6. Compute the Composite Lifting Index for the job according to the following formula :

CLI = STLI1 + (FILI2 + (FILI3 + (FILI4 + (FILI5

where :

(FILI2 = FILI2 x 1 – __1__ FM1,2 FM1

(FILI3 = FILI3 x 1 – 1___ FM1,2,3 FM1,2

(FILI4 = FILI4 x 1 – 1___ FM1,2,3,4 FM1,2,3

(FILI5 = FILI5 x 1 – 1____ . FM1,2,3,4,5 FM1,2,3,4 . . . . (FILIn = FILIn x 1 – 1_______ FM1,2,3,4,5,…n FM1,2,3,4,5,…(n-1)

Revised 1991 NIOSH Lifting Equation

1 Section 2: The Revised NIOSH Lifting Equation Page of 11

Prepared by Bich Huynh – Ergonomist, VWA, Melbourne

Note: the NIOSH Applications Manual is a significant reference relevant to this page.