In order to light the streets of the Metropolis (when Sherlock Holmes was written, the word Metropolis was capitalized and only applied to London) the flames of the street lamps had to be produced with a stable gas not likely to explode and that was relatively cheap to produce. The inventive Victorians rose to the necessity and used coal gas, a cheap derivative of coal used to light the numerous streets of the growing city. Coal gas was manufactured usually fairly close to the area where it would be used. Even though it was not highly volatile, coal gas could still explode. But it had a darker aspect, too. One of the chief results of the process of distillation was ammonia which was released into London’s air. When mixed with the other byproducts of industrialization, it was quite toxic. The “fogs” of Victorian London were a fairly lethal mixture that included ammonia and sulphuric acid (another byproduct of the process of producing coal gas). What a deadly cocktail! It was especially bad in the neighbourhoods occupied by the coal gas plants, areas which usually included high density homes of the poor.
When, in novels of the period, we read about the “bad night air” of London it was “air” that could scorch people’s lungs and make them sick. The air of London was usually ten degrees warmer than the surrounding countryside because of the greenhouse gases. All in all, it was not a good place to breath.
So, in addition to the to the streets and “atmosphere” of darkest London where crimes were hidden from sight and so committed with impunity, the air itself was a silent killer.