Summers are getting hotter, and this year temperatures have regularly been over 30 degrees Celsius. This is uncomfortable for many, especially those working outside for long periods. A maximum temperature for outdoor working would mean that workers couldn't be required to work in these conditions.
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The health impacts of such warm temperatures, and long exposure to the sun are already known. The law places a legal obligation on employers to provide a 'reasonable' temperature in the workplace, but there is no legal maximum where workers cannot be required to work outdoors. This means many workers are required to work outdoors, doing manual labour, in unreasonable temperatures.