NIOSH Lifting Equation: Ergonomics Guidelines for Safe Manual Lifting

Introduction

At the outset it is important to understand the scope of the effort here. This is not merely an attempt to calculate how a person should perform their work — it is also a review of the methods used to determine appropriate limitations on physical labor. When those limitations are considered, different methods exist for arriving at decisions, and more specifically, three distinct systems are available. Only one of them is legally enforceable, but that may not always be what is best for the individual worker. One must look at the welfare of the person performing the job and find a method through which both worker and employer may benefit. At the same time, this is not simply an act of mercy — the decision must also be economically viable for the employer to continue their business. Otherwise, both employer and employee will lose their economic footing. A full discussion of the available methods is therefore required, so that any decision reached is grounded in evidence rather than personal judgment. Decisions in this area remain somewhat open-ended, as there are many interrelated factors that require further study before a fully definitive answer can be offered. What may ultimately be required is a comprehensive site-specific study of the problem before any final decision is made.

There is no doubt that physical work is essential for many individuals' livelihoods, and it is among the oldest forms of human labor. As the world's most economically advanced nation, the United States has a particular responsibility to ensure that those who perform physical work are treated justly. The methods for doing so will be discussed in the sections that follow.

The worker whose case is examined here is employed at a company involved in receiving products from a major manufacturer and distributing them to other organizations for retail sale — in short, a wholesaler of consumer goods. His job involves receiving material as it arrives on trucks, storing items in their designated locations within the warehouse, and then loading them onto trucks for delivery to recipients. The job is essential because goods arrive from the manufacturer in single-item lots, while outbound shipments consist of multiple different items. In practical terms, his job is lifting and lowering material throughout the workday.

The Three Lifting Guidelines

Three applicable guidelines exist for this type of work. They are: (a) the National Institute for Occupational Safety and Health (NIOSH) Lifting Equation of 1991; (b) the proposed American Conference of Governmental Industrial Hygienists (ACGIH) Lifting Tables; and (c) the Snook and Ciriello Psychophysical Tables, as published in Liberty Mutual Research, 1991. When these different guidelines are compared, there are more differences than similarities among them. Those differences create difficulty in evaluating lifting and lowering activities, making it hard to determine the actual risks associated with such tasks. Each will be examined in detail before a course of action is recommended. (Faville; Shulenberger, 2004)

For this type of operation, the relevant document is the Applications Manual for the Revised NIOSH Lifting Equation, authored by Waters TR, Putz-Anderson V, and Garg A, of the National Institute for Occupational Safety and Health. This document was published in January 1994 and is available through the U.S. Department of Commerce. According to the manual, difficulties associated with jobs like this one arise from repetitive elements, the forces required, and awkward postural demands. The greatest risk is to the lower back. Problems arise specifically from sorting and handling packages, delivering beverages, and manually lifting weights exceeding 10 pounds — particularly in a productive work context. The effects can potentially affect any part of the body.

The specific risks for this worker include lifting more than 75 pounds; lifting more than 55 pounds more than 10 times in a single day; lifting more than 25 pounds below knee level more than 25 times a day; lifting at arm's length; and engaging in push-and-pull activities exceeding 20 pounds of force. This last figure is considered equivalent to pushing a 65-pound box across a tiled floor, or pushing a shopping cart loaded with 200 pounds. These activities are performed for more than two hours per day. (Analysis Tools for Ergonomists)

This model is based on biomechanical, physiological, and epidemiological data. The analysis rests on two fundamental assumptions: (1) lifting and lowering tasks carry equivalent risks for lower back injury, and (2) the static friction between the worker and the floor surface is at least 0.4 coefficient of static friction, based on the interaction of shoe sole and floor surface. It is important to note that this model does not account for unpredictable conditions such as unexpectedly heavy loads, slipping, or falling. It also does not address one-handed lifting, lifting while seated or kneeling, lifting in a constrained or restricted work area, or lifting items wider than 30 inches. Notably absent from these guidelines is any consideration of the worker's sex — the same weight limits apply to both male and female workers. (Faville; Shulenberger, 2004)

Development and Progress of the NIOSH Guidelines

Lower back pain and injuries have long been attributed to manual lifting activities, and they remain the leading occupational health and safety issue in preventive medicine. Many programs directed at both workers and job design have attempted to address this problem, yet work-related lower back injuries continue to account for a substantial portion of national suffering and economic loss. The scope of this problem was captured in a Department of Labor, Bureau of Labor Statistics report titled "Back Injuries," published in 1982. Its findings aligned with existing workers' compensation data, and it concluded that "injuries to the back are one of the most common and most costly types of work-related injuries" — a finding echoed by the National Safety Council in 1990. (Applications Manual for the Revised NIOSH Lifting Equation)

According to the Department of Labor report, back injuries account for nearly 20% of all workplace injuries and result in approximately 25% of annual workers' compensation payments. While that report is now dated, a 1990 National Safety Council report confirmed that overexertion remains the most significant cause of occupational injury, accounting for 31% of all injuries. The back was identified as the most frequently injured body part, constituting approximately 22% of the 1.7 million injuries occurring each year — and the single largest contributor to workers' compensation costs. (Applications Manual for the Revised NIOSH Lifting Equation)

The National Institute for Occupational Safety and Health recognized the problem more than a decade before the 1991 revision and published the Work Practices Guide for Manual Lifting in 1981. That report summarized the published literature on back injuries prior to 1981, and introduced both analytical procedures and a lifting equation for developing a recommended weight for two-handed, symmetrical lifting tasks. The goal was to minimize the likelihood of low back injury from manual lifting. This analysis was tied to the concept of the Action Limit — the recommended weight derived from the lifting equation.

Further progress came in 1985, when NIOSH convened an ad hoc committee of experts to review the existing literature on lifting, including the 1981 Work Practices Guide. The resulting literature review was released in a document titled Scientific Support Documentation for the Revised 1991 NIOSH Lifting Equation: Technical Contract Reports, May 8, 1991. (Applications Manual for the Revised NIOSH Lifting Equation) This update contains information on the physiological, biomechanical, psychophysical, and epidemiological aspects of manual lifting. Based on its findings, the ad hoc committee recommended new criteria for defining the lifting capacity of healthy workers, which were then used to formulate the revised lifting equation. (Applications Manual for the Revised NIOSH Lifting Equation)

It is important to recognize that the NIOSH lifting equation is only one tool in the broader effort to prevent work-related low back pain and disability. Lifting itself is only one of several causes of work-related low back problems. Other contributing factors include whole-body vibration, static postures, prolonged sitting, and direct trauma to the back. (Applications Manual for the Revised NIOSH Lifting Equation) Psychosocial factors, appropriate medical treatment, and overall job demands are also significant in the onset of acute low back pain and may contribute to the development of chronic disabling conditions. (Applications Manual for the Revised NIOSH Lifting Equation)