Outdoors, when very polluted, you can see and taste and smell it. But indoors, you often can’t detect what’s there.

Chris Birch used to dread late afternoon meetings at the old Stretford office of Hilson Moran, a Manchester engineering consultancy.

“You’d be sat in a meeting for three hours, feeling slightly headachy, tired and stuffy,” says Birch, the company’s head of sustainability.

The windows in the conference rooms (and the rest of the office) were kept shut all year round; in winter to prevent the cold getting in and in summer to stop particulates, carbon dioxide and nitrogen dioxide from car exhaust fumes drifting in from nearby heavy traffic.

But sealing the building meant every breath an employee took during those long meetings would raise the carbon dioxide level in the room, causing drowsiness and headaches.

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