That's an interesting point, my bet would be if you see white /yellow smoke you are just smouldering with no flaming combustion, black smoke then the fire is well air starved, blue smoke the fire is quenched or not up to temperature. I'd love to see sampling done if you truly cannot see any smoke but would say it's as clean as you're going to get.
Yes there will always be particulates but if you saw that Economist graphic you will note there is always a background particulate level that is not attributed and since 1990 this has become more significant as other sources have been controlled (especially look at how agriculture's contribution dropped so dramatically, presumably following the straw burning ban).
Consider also that all particulates are not equally toxic, these surveys do not discriminate whether particles are of a more damaging type, like asbestos, infectious spores or containing PAHs, also the risks are not directly proportional to the concentrations, they go up dramatically if you smoke for instance.
Now I accept that breathing in particulates is damaging, I once worked out that over my lifetime I have probably inhaled over 2 grams but I am approaching my biblically allotted span. I almost certainly have loss of lung function, after all I sat behind a dies which emitted blue smke, from glazed bores I suspect)and worked near many bonfires. This is the worry to me about how theses cause and effect are being attributed. premature deaths and Disability-Adjusted Life Year effects are based on a cohort of all who have died up till now. Many of these people will have lived, breathed and existed in industrial pollution from years back, you can see from the graphic cited above that even since 1990 we have made significant improvement and before that we had the clean air act. I hardly ever see dark smoke nowadays and when I do it's often a shady industrial yard illegally burning off old tyres etc.