Heart rate

Heart rate measuring equipment is readily available, easy to use and interpret eg portable heart rate monitors and watches.  The devices are useful as they do not interfere with performance and are not as cumbersome as an electrocardiogram.  Certain monitors allow for the storage of heart rate over time to be made, with the retrieval of the data using software making the examination of the worker’s heart rate over a shift or 24 hour period possible.  The disadvantage of these devices is that close proximity (about 1 metre) of one monitor to another may interfere with their performance.

Alternatively the worker, supervisor or occupational health professional could easily take manual measurement of heart rate through pulse detection at the wrist or neck and they need to be trained in taking manual measurements.

In situations where thermal strain is very high, (as predicted by the method described in ISO 7933), the accompanying measurement of core temperature is necessary.  In addition, the system used should allow real time monitoring of heart rate during exposure.  This should ideally be determined either by an individual test or predicted by the following equation:

1               HRL = 185 - (0.65*A)

Where HRL = Heart Rate Limit, and A = Age.

The HR limit value in the workplace should not exceed the maximum value of the person reduced by about 20bpm though the use of this value may represent a significant risk for some subjects.

In accordance with the maximum limit of 39°C set for the core temperature, the maximum limit for the increase in HR from thermal origin is set at 60 bpm.  This applies in the same circumstances and especially when medical supervision and continuous monitoring are provided.

Sustained heart rate of less than 160bpm if worker is younger than 35 years old

Sustained heart rate of less than 140bpm for workers older than 35 years old.

Whatever method is used it is important not to consider any individual measure (symptoms, core temperature, heart rate) individually but where possible to consider all three. 

This is the final part of the risk assessment and control strategy.  It can only be used once an appropriate heat stress risk assessment has been carried out using current heat stress standards (eg BS 27243 and BS 12515) and once all appropriate heat stress control measures have been employed.  The following are some of the factors which should be considered:

• Use of physiological monitoring equipment and data interpretation must only be performed by competent staff.  Improper use and/or misinterpretation of data may significantly increase the risk of heat stress due to erroneous results;
• Real-time observation of data is required for health monitoring of workers in moderate to high heat stress situations;
• Care should be taken to ensure that the operating range (ie the environment in which the equipment will be used) is within the equipments design limits.  Where environmental conditions are outside the manufacturer’s stated operating range, adequate insulation of equipment is provided;
• The measures taken must be appropriate to the thermal environment and the risk of heat stress (see BS EN ISO 9886 (2001));
• All equipment should be regularly calibrated (as per manufacturer’s instructions) with a recognised calibration agency or laboratory;
• Any equipment monitoring workers wearing PPE should be validated prior to its use.  Specifications as to its accuracy, range, sensitivity, operating environment, etc. should be supplied.  This sort of system may be used as a monitoring system and when validated as a system for establishing work/rest schedules etc, when used by an expert in human thermal physiology.    

Medical supervision of people exposed to heat must be carried out to assess suitability and identify any reasons for not being permitted to work in that environment.  When this occurs it should be by suitably qualified medical personnel on a regular basis with substantial experience of occupational heath factors associated with working in these environments (see BS EN ISO 12894 (2001))

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