What does the red mean?

[v0031]

The worst effect of the smog, however, was the respiratory distress it caused in humans and animals, including difficulty breathing and the vomiting of phlegm. One of the first noted victims was a prize cow that suffocated on December 5. An unusually high number of people in the area, numbering in the thousands, died in their sleep that weekend.
It is difficult to calculate exactly how many deaths and injuries were caused by the smog. As with heat waves, experts compare death totals during the smog to the number of people who have died during the same period in previous years. The period between December 4 and December 8 saw such a marked increase in death in the London metropolitan area that the most conservative estimates place the death toll at 4,000, with some estimating that the smog killed as many as 8,000 people.
On December 9, the smog finally blew away. In the aftermath of this incident, the British government passed more stringent regulations on air pollution and encouraged people to stop using coal to hear their homes. Despite these measures, a similar smog 10 years later killed approximately 100 Londoners.

[publicunofficial]

Heat waves = Long period of hot weather

So the sentence is saying deaths caused by heat waves and deaths caused by smog are recorded by the same method.

[v0031]

The smog became so thick and dense that by December 7 there was virtually no sunlight and visibility was reduced to five yards in many places. Eventually, all transportation in the region was halted, but not before the smog caused several rail accidents, including a collision between two trains near London Bridge.