Spark plugs don't appear in every four stroke engine, as a diesel motor employs high temperatures caused by compression to achieve the fuel ignition. The idea of an engineer called Rudolf Diesel, near the XX century, studying the —dangerous— properties of the fuel ignition with this technique.
Today multiple manufacturers provide diesel alternatives, and some of them breaking limits with low consumption rates per unit of distance. It's not the first time that we talk about a turbo diesel engine that today has the record in low consumption, which is curious since the hydrocarbon of the fuel has less octanes.
The fuel, then, is not exactly the same, and it's also intensely used for heating purposes. Another vast source of air pollutants, and specific ones to add in the already long list. They are dangerous, the cause that increases its price due to taxes in several places.
During some time, the filter to avoid the pollution was an option, leaving to the client the decision of harming the public air or not. After the current realities related with the atmosphere and health, now it's obligatory in the "state of the art" talking about development.
Curious sentences like "it's better for your pocket" are against "it's worse for your lungs and health, and not only yours".
While these filters provide a great protection, what escapes is perhaps the most dangerous by mere logic. Some people say 99% efficiency, and leaving that 1% aside and multiplying it for one hundred million, there's enough to be breathing them frequently!