The Great Smog, 1952: the hazard London had a nickname for

London called it a pea-souper. There were songs about it and jokes in the novels. Smog was not an emergency in December 1952, it was weather, and the city had been living inside it for a hundred years. Then an anticyclone parked over the capital, trapped four days of coal smoke under a lid of cold air, and the undertakers ran out of coffins.

Nothing new happened that week, and that is the point

That is the part worth sitting with. No factory exploded, no new chemical was released, no process changed. The city burned exactly what it burned every winter. The only variable was that the smoke could not disperse, and a hazard everyone had learned to live with revealed what it had been doing at a slower rate all along.

About four thousand deaths were attributed to it at the time. Later analysis put the real figure substantially higher. The government's initial position was that it had been an unusually bad fog.

The clinical picture was specific rather than vague. Hospitals filled with people in acute respiratory failure, the worst affected being the elderly, infants and anyone with existing lung or heart disease. Undertakers ran out of coffins and florists ran out of flowers. Cattle at the Smithfield show were found suffocated in their stalls, which is the detail that persuaded people the air itself had done it, because nobody could argue the cattle had been smoking.

What the Clean Air Act actually did

The Clean Air Act 1956 did something quietly radical: it made ordinary, legal, domestic and commercial behaviour into something that required permission. Local authorities could declare smoke control areas where only authorised fuels could be burned. Chimney heights became a regulated matter. Emitting dark smoke became an offence, with the operator of the plant carrying it.

It took three and a half years from the smog to the Act, against an economy built on coal and a public who saw the fuel bill before they saw the risk.

Where this lands in a modern portfolio

Air quality is now a cost with a meter on it. Clean air zones and low emission zones charge non-compliant vehicles daily — London's ULEZ runs at £12.50 a day, every day of the year — which lands on your fleet, your contractors and every delivery made to your buildings. Planning consent for a plant replacement in an air quality management area can require dispersion modelling before anyone signs. Boilers and generators sit inside emissions limits, and the standby generator that has always smoked for the first minute of its monthly test is a compliance question every time it runs.

The indirect cost is larger and quieter. Occupiers now ask for indoor air quality data in tenders, valuers increasingly ask what a building's operational emissions look like against the trajectory it needs, and a building that cannot answer either question starts competing on price alone. The Clean Air Act made the invisible a legal object. The market has since made it a commercial one.

You do not burn coal, but you probably hold combustion plant: boilers, standby generators, CHP units, occasionally a biomass installation somebody specified for the carbon numbers. Those sit inside this lineage — chimney heights, emissions limits, and in many places a local air quality management area that turns a plant replacement into a planning conversation.

The modern version of the 1952 mistake is the generator test. It runs monthly, it produces visible smoke for the first minute, it has always done that, and nobody has ever measured it or asked whether the flue discharges next to a fresh air intake or a residential window.

An inversion is a lid, and the city kept burning fuel under it

The mechanism was meteorological. An anticyclone settled over the Thames basin and produced a temperature inversion: a layer of warmer air sitting above colder air at ground level, which suppresses the vertical mixing that normally carries smoke away. With no wind to move it sideways and no convection to lift it, four days of emissions accumulated in a few hundred feet of air.

What accumulated mattered as much as how much. Domestic coal burned at low temperature in open grates produces high particulate and high sulphur dioxide. In the moist air of the fog the sulphur dioxide oxidised to sulphuric acid on the surface of the particles, so the population was not inhaling smoke so much as an acid aerosol with a particle size small enough to reach the deep lung. That is why the deaths were respiratory and cardiac, and why they were concentrated among people whose lungs and hearts were already compromised.

What the Act regulated, and what your plant sits under now

The Clean Air Act attacked the mechanism from three directions: the fuel, through smoke control areas where only authorised fuels could be burned; the discharge, by making dark smoke an offence with the operator carrying it; and the dispersion, by regulating chimney heights so that what was emitted was released high enough to mix rather than settle.

That third one is the one that still catches building owners out. Dispersion is a design calculation, and it depends on discharge height, efflux velocity, building downwash and what sits nearby. Replace a flue like for like on a building that has since acquired a taller neighbour, or a residential conversion across the street, or a fresh air intake two floors up, and the calculation that justified the original installation no longer describes your site.

Your 60-second self-check

• Do you know the discharge height and location of every flue on your estate, and what sits within ten metres of it?
• Is any of your combustion plant in a smoke control area or an air quality management area, and does your maintenance contractor know that?
• When a plant item is replaced like for like, does anyone check whether the emissions position has changed since it was installed?

A hazard everyone has stopped noticing has no owner, and no owner means no control. The free 5-minute Director's Risk Check maps which duties and deadlines apply to your portfolio, each with its official source, so you can see where the exposure sits before someone else finds it for you. Run it at the Director's Risk Check, or send it to whoever holds the files. The hazard nobody has looked at in years is the one with no owner, and no owner means no control.

Herman's verdict

Four days of ordinary weather killed thousands of people doing nothing unusual. The Clean Air Act's achievement was not technical — it was taking a thing the whole city had normalised and giving it a name, a limit and somebody to answer for it.

— Herman