It is essential, in a modern world, to limit the pollutants. That is the situation: no ifs, no buts. Determine at what distance you can dispense with breathing apparatus in the common situation of a ruptured bag. It's not hard. ■

Stack heights are the issue. When a bag fails the airflow rockets up the stack and what goes up might come down. Surrounding air cleanliness will determine the necessity of PPC provision & operator access must not be compromised at any time. I have worked in some disgusting sites & survived. Zambia had some air lurking in the fumes & dust. So did SAMANCOR & McKechnies on the Reef. I remember a case in Reckitts Derby, UK where the incompetents built a plant for making toilet blocks. It was a fancy job which used the stuff that kills more than three usual 99%. Commissioning highlighted a mass balance deficit just equal to the mass of the +99% stuff. Subsequent examination revealed that the lethal stuff was getting heated by the pneumatic conveying lines & the vapour state was escaping out of the filters. There was a serious risk that the vapour would sterilise the surrounding population. In the best multinational approach Reckitts shut down the entire site and kept silent.

You simply have to make sure that workings are safe at all times. I've seen a statement in these forums "Of course somebody is going to get killed." Maybe it was that writer.

Standards for hazards are just that. Forget engineering. Krakatau Steel approved a reactor plant extension where the open reactor outlets had a shrouded landing directly overhead. Another masterpiece of MAN at work.

It hardly matters what you consider to be a normal nose elevation if it turns out to be at ground zero when someone is fuming. ■

If the pollutant is gas or vapour the bags are useless, like they were at Reckkits. Dilution there is worse. Containment is the goal, always. Isn't that why filters exist?

Solids dispersion is almost as bad but Escom, Saath Efrika, took the biscuit, crumbs & all, by doping the coal fuel with radioactive isotope and then tracking the power station smoke with an aeroplane fitted with Geiger counters.

It's nice to be old and looking at engineering as a source of incredulity.

There was a Taiwanese directive for power plants that whatever got into the plant stayed in the plant. It's well worth looking into. ■