Lifting and Heat Stress Limits

Lifting and Heat Stress Limits

Lifting Equation and Heat Stress Limits: Method, Tables and Sources

What the tool answers

Two questions a safety or ergonomics consultant is asked on a site visit: is this manual lifting job within the weight a healthy workforce can handle over the shift, and is this combination of heat and work within the limit for the people doing it. The lifting part is the Revised NIOSH Lifting Equation as printed in the Applications Manual (NIOSH Publication 94-110); the heat part is the wet-bulb globe temperature comparison of the NIOSH criteria document (Publication 2016-106) with the National Weather Service heat index beside it. Both are screening methods for healthy workers; neither replaces a medical or a detailed biomechanical assessment.

The Revised NIOSH Lifting Equation

The Recommended Weight Limit is the load that nearly all healthy workers could lift over a substantial period (up to 8 hours) without an increased risk of lifting-related low back pain. It is a load constant reduced by six multipliers, each 1.0 in the ideal lift and smaller as the lift departs from it:

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

H is the horizontal distance from the mid-point between the inner ankle bones to the mid-point of the hand grasps, V the height of the hands above the floor, D the vertical travel between origin and destination, A the angle between the line to the load and the worker's mid-sagittal line. The two unit forms are both printed in the manual and are not exact conversions of each other (51 lb is 23.13 kg; 25 cm is 9.84 in), so a job analysed in both systems differs in the second or third figure. The tool uses the US form in USCS and the metric form in SI.

Frequency multiplier. F is the average number of lifts per minute over a 15-minute sample; below 0.2 it is taken as 0.2. The duration category is short (work of 1 hour or less followed by recovery of at least 1.2 times the work time), moderate (over 1 and up to 2 hours, recovery at least 0.3 times the work time) or long (up to 8 hours with normal rest allowances). A recovery period shorter than required is ignored and the work periods are added. When lifting comes in bursts within the 15 minutes, F is the number of lifts in the 15 minutes divided by 15 (the manual's special procedure), provided the actual rate does not exceed 15 per minute. The table, with V below or at and above 30 in (75 cm):

Between rows the tool interpolates linearly, as the manual does for 2.4 lifts per minute in its Examples 7 and 8; past the last non-zero row of a column the multiplier is zero.

Coupling multiplier. Good coupling 1.00 at any height; fair 0.95 with the hands below 30 in (75 cm) and 1.00 at or above; poor 0.90 at any height.

Origin and destination. The limit is computed at the origin of the lift. When the worker has to re-grasp the load near the destination, hold it there for a moment, or position or guide it, the manual calls this significant control at the destination: the limit is then computed at both ends, each with its own H, V, A and coupling multiplier, and the lower value governs.

Lifting Index. LI = load / RWL. The manual's position is that jobs with an index above 1.0 pose an increased risk for some fraction of the workforce and should be redesigned to 1.0 or less, and it reports the view of experts that nearly all workers are at increased risk above 3.0. The index ranks jobs; it is not a probability of injury.

Multi-task jobs

When the tasks of a job differ (tiers of a pallet, several shelves, cartons of different size), averaging the task variables hides the bad ones. The manual's procedure keeps each task separate:

1. FIRWL, the frequency-independent limit of each task: the equation with FM = 1, at the governing end.
2. STRWL = FIRWL × FM, the limit of the task as if it were the only one performed.
3. FILI = maximum load / FIRWL, the strength demand of one lift; STLI = average load / STRWL, the task with its own frequency.
4. The tasks are renumbered by decreasing STLI (ties: the higher frequency first), and the Composite Lifting Index is CLI = STLI₁ + Σ FILI_n (1 / FM_1..n − 1 / FM_1..n−1), where FM_1..n is read from the frequency table at the sum of the frequencies of tasks 1 to n with that task's origin height and the job's duration.

Each added task therefore raises the index by what the extra frequency costs that task. With one row in the table the tool gives the single-task result; with several it adds the multi-task tables. A FILI above 1.0 marks a lift that is too heavy even once; an STLI above 1.0 marks a task that is too demanding on its own.

Rounding, and what the manual's examples show

The manual's worksheets round every multiplier to two places, weight limits and indices to one. The tool's default does the same for the multipliers and carries the other values unrounded, so its indices can differ from a worksheet in the last printed digit. The test script reproduces all ten example problems of the manual and their redesigns. Three things found on the way: Table 1 of the manual prints 0.46 and 0.44 for H = 22 and 23 in where 10 / H rounds to 0.45 and 0.43, and Table 3 prints 0.86 at D = 130 cm where the equation gives 0.85 (the tool uses the equations; Examples 1 and 8 use the printed cells, which moves their limits by about 2 %); Example 4 prints an origin index of 0.8 for 26 / 34.9 = 0.745; and the Composite Lifting Index of Example 10, printed 3.6, is the sum of rounded worksheet entries (3.67 unrounded).

The manual's redesign suggestions (its Table 8) are printed for the lowest multiplier of each task: bring the load closer (HM), raise or lower the origin or destination (VM), shorten the travel (DM), remove the twist (AM), reduce the frequency or duration or lengthen recovery (FM), improve the handholds (CM), and remove the need for control at the destination when the destination governs.

Limits of the equation. It applies to two-handed lifting with little other manual handling, on a floor with good footing, at moderate temperature and humidity (about 19 to 26 °C and 35 to 50 %). It was not designed for one-handed lifting, lifting while seated or kneeling or in a restricted space, unstable loads, wheelbarrows or shovelling, or high-speed lifting. Jobs that rotate workers between different lifting tasks through the shift (sequential lifting) and jobs whose loads and positions change from lift to lift (variable lifting) have their own later NIOSH procedures, which are not in this tool; pushing, pulling, carrying and hand-arm vibration are separate assessments.

Wet-bulb globe temperature

The WBGT combines humidity and air movement (through the natural wet-bulb temperature t_nwb), radiant heat (through the globe temperature t_g) and, in sunshine, the air temperature t_a:

indoors, or outdoors without a solar load: WBGT = 0.7 t_nwb + 0.3 t_g
outdoors with a solar load: WBGT = 0.7 t_nwb + 0.2 t_g + 0.1 t_a

The natural wet bulb is an unaspirated wetted thermometer exposed to the radiation and wind of the place; it is not the psychrometric wet bulb of a weather report, and the tool does not estimate a WBGT from weather data: enter the three readings, or the reading of a WBGT instrument, for each segment of the hour. Because the weights add to one, the equations hold in °C or in °F.

Metabolic rate

The limits depend on the metabolic heat the work produces, as a one-hour time-weighted average for a 70 kg worker. The criteria document classes work as resting (up to 100 kcal/h, 117 W), light (below 200 kcal/h, 233 W), moderate (201 to 300 kcal/h, 349 W), heavy (301 to 400 kcal/h, 465 W) and very heavy (401 to 500 kcal/h, 580 W); the workload select uses the upper end of each class, or the watts can be typed (1 kcal/h = 1.163 W). The document notes that estimates from tables may be in error by 10 to 15 % or more.

NIOSH recommended limits

acclimatized workers: REL = 56.7 − 11.5 log₁₀ M
unacclimatized workers: RAL = 59.9 − 14.1 log₁₀ M   (°C-WBGT, M in watts)

Both the WBGT and M are time-weighted over the hour, rest periods included: WBGT = Σ(WBGT_i t_i) / Σ t_i and M = Σ(M_i t_i) / Σ t_i. The exposure is within the limit when the time-weighted WBGT, plus the clothing adjustment, does not exceed the limit at the time-weighted M. At 349 W the equations give 27.5 °C and 24.1 °C; the document's own example reads 27.8 °C and 25 °C from its figures. The figures also draw curves for 45, 30 and 15 minutes of work per hour, for which no equation is printed; the tool applies the two printed equations to the time-weighted hour. The ceiling limits of the earlier edition were removed from the 2016 document and are not used.

Clothing. The limits assume one layer of ordinary work clothing. For other ensembles an adjustment is added to the measured WBGT. The select fills the values of the document's Table 3-2 (cloth coveralls 0, double-layer cloth 3, SMS coveralls 0.5, polyolefin coveralls 1, limited-use vapor-barrier coveralls 11 °C-WBGT), which the document adapts from ACGIH; any other value can be typed. For fully encapsulating impermeable suits the document says the WBGT is not the right measure.

Work and rest schedule. The tool finds the largest share of the hour that can be spent in the work condition, the remainder being spent in the recovery condition, such that the time-weighted pair sits on the limit. The work condition is the minute-weighted average of the segments marked as work; the recovery condition is the average of the segments marked as rest, or, when none is entered, rest (117 W) at the work location. A cooler rest area therefore raises the permitted work time twice, through the WBGT and through the metabolic rate.

Acclimatization. The document's plan: new workers no more than 20 % of the usual duration of work in the heat on day 1 and 20 % more each day; workers with previous experience 50, 60, 80 and 100 % on days 1 to 4. Full acclimatization takes 7 to 14 days and is largely lost after a week or more away.

Heat index

The National Weather Service heat index is Steadman's apparent temperature for a person walking in shade in a light wind, fitted by Rothfusz (1990) as a regression in the air temperature T (°F) and the relative humidity RH (%):

HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH²

The tool follows the Weather Prediction Center's procedure: the simple formula HI = 0.5 [T + 61 + 1.2 (T − 68) + 0.094 RH] is averaged with T, and when that is below 80 °F the simple formula is the answer; otherwise the regression applies, less [(13 − RH)/4] √{[17 − |T − 95|]/17} when RH is below 13 % and T is between 80 and 112 °F, plus [(RH − 85)/10] [(87 − T)/5] when RH is above 85 % and T is between 80 and 87 °F. The Service's chart (80 to 110 °F, 40 to 100 %) is the plain regression. Classes: Caution 80 to 90 °F, Extreme Caution 90 to 103 °F, Danger 103 to 124 °F, Extreme Danger 125 °F and above; full sunshine can add up to 15 °F. The index carries no workload and no clothing, which is why occupational limits are written in WBGT.

Not covered

An estimate of the WBGT from weather data (the Liljegren model), the ACGIH threshold limit values and action limits, ISO 7243, the screening tables of the OSHA Technical Manual, physiological monitoring (heart rate, core temperature), the sequential and variable lifting indices, push and pull force limits and hand-arm vibration.