Thermal Work Limit Chart

Last Updated: December 31st, 2022/Views: 6344/1.7 min read/
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Thermal Work Limit (TWL)

Working zones

Interventions

Rehydration Schedule

Work-Rest Schedule(minutes)

Low Risk

TWL: 140 – 200

No limits

on self-paced work for educated hydrated workers

Light Work

0.6 – 1

Litres/Hour

Safe for all continuous self-paced work

Medium Risk

TWL: 115 – 140

Additional precautions required

 

  • Practical engineering control measures to reduce heat stress should be implemented, e.g. provide shade, improve ventilation etc.;
  • Avoid Lone-Working;
  • No unacclimatized person to work;
  • Ensure adequate fluid intakes appropriate for type of work;
Light Work

1-1.2 Litres/Hour

Safe for all continuous self-paced work
Heavy Work

>1.2 Litres/Hour

Continuous paced work

45 Work – 15 Rest

High Risk

TWL: < 115

Strict Work – Rest Cycling required

  • Lone-Working is prohibited;
  • No Unacclimatized person to work;
  • High Risk Induction required emphasizing hydration and identifying sighs of Heat Stress;
  • Provide sufficient drinking water (2L per person) to job site all the time;
All Work

>1.2. Litres/Hour

Light Work

45 Work – 15 Rest

Heavy Work

20 Work – 40 Rest

TWL LEGEND

  1. Self-paced work – Workers must be allowed to adjust their work rate according to environmental conditions. Paced work is when the work rate is not under the worker’s controls
  2. Unacclimatized workers are defined as new workers ot those who have been of work for more than 14 days due to illness or on vacation leave (in a cool climate area)
  3. Light work – sitting or standing, light arm work
  4. Heavy Work – carrying, climbing, lifting, pushing, whole body work

Note: At high workloads and or thermal stress, sweat rate exceed 1.2 Litres/Hour. Increasing fluid intake much above this level is not practical due to gastric discomfort as the upper limit for gastric absorption is ~ 1.5 Litres/Hour so control solutions to improve thermal conditions should be implemented in addition to providing adequate hydration to replace sweat lost.

Additional factors contributing to heat illness

  • Wearing of additional PPE e.g. chemical suit; RPE
  • Heavy or forced paced workload
  • High level of task complexity e.g. precision operation
  • High level of environmental noise or vibration
  • Body Mass Index (BMI) greater than 30
  • Aging worker / Unfit worker
  • Medical condition or medications taken.

2 Comments

  1. shabeer October 15, 2022 at 6:58 am - Reply

    Asalamualikum,
    The above thermal chart index details from which standatd uk/Usa/oman?

    • caspianexplorer
      caspianexplorer October 16, 2022 at 6:31 am - Reply

      Wa aleykum Salam
      Dear Shabeer,

      This is the chart used by the Industrial Hygienists in PDO, at least it was back in 2019.

      Hope this helps,
      EntirelySafe.com team

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