The city under a lid

01 / The briefing

Begin with the question.

London's December 1952 smog presents a problem of interacting causes. Smoke sources existed before the episode, and fog was familiar, but familiarity did not make the combination safe. Your investigation separates three questions: what entered the air, what prevented it from dispersing, and what evidence distinguishes an ordinary visibility problem from a dangerous pollution event. The evidence cards summarize official historical material rather than contemporary medical guidance. Think of the atmosphere as a changing container. The same flow into a large, well-mixed space and a shallow, poorly ventilated space can produce very different concentrations. A convincing account must explain why the episode developed, why it ended, and why waiting for better weather was not a sufficient prevention policy.

The timeline

1. 1952-12-04

   Daily smoke concentration cited by London City Hall is 490 micrograms per cubic metre.

2. 1952-12-05

   The Great Smog begins during cold, settled weather.

3. 1952-12-07–08

   The cited daily smoke concentration reaches 4,460 micrograms per cubic metre.

4. 1952-12-09

   A change in weather disperses the smog.

5. 1956-07-05

   The Clean Air Act receives Royal Assent.

A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.

02 / The evidence

What does the record show?

Read each exhibit before settling on an explanation. The numbered source links below every exhibit lead to the record behind it.

03 / Your reasoning

Follow the strongest explanation.

Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.

04 / The findings

The chain behind the outcome.

The cause was a conjunction: substantial combustion emissions and weather that restricted their dilution. A temperature inversion is not itself a source of smoke, and smoke alone does not explain why concentrations increased so sharply at that moment. Combining the two supplies the missing mechanism. The later weather change removed the immediate atmospheric trap, while legislative measures addressed emissions over a longer timescale. This distinction matters when interpreting success: an episode can end naturally without its underlying vulnerability having been removed. Mortality estimates vary with the population and follow-up period, so this case does not turn one estimate into a definitive tally. The investigation is historical. Its general lesson is to separate source, transport and exposure, and to choose interventions that interrupt the relevant link rather than merely improving the appearance of the problem.

Take the method with you

• A familiar source can become dangerous when dispersion changes.
• Separate episode-ending weather from long-term prevention.
• Use measurements, not visibility alone, to characterize pollution.

You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.