hot weather thoughts

Here in the UK the weather is unusually hot for this time of year. Many offices have air-conditioning or fans, and of course many don't. How hot does it have to be before the building becomes an unsafe working environment? How tough should you be with your team about taking time off, or extending their lunch breaks for some sunbathing in the park? It's sometimes a hard decision to make, between not wanting to seem a killjoy and wanting staff to maintain a professional attitude in spite of the weather.