Antibiotic resistance is a thing not because antibiotics are a molecular shredder that chews up the bugs. It's because they're some kind of metabolic poison.
I might be able to increase my resistance to Iocaine powder by slowly dosing up. But I can't increase my resistance to Andre the Giant tearing me limb from limb.
I believe they were asking specifically that. How do you ensure that?
In other words, how do you ensure all get the correct (lethal) dose?
Some may be partially hit. It would be very hard to tell, no?
If you put these in ductwork then you need to look at the flow rate through the duct and the required light flux and then ensure that the supplied flux is well in excess of that.
Remember we're not talking about your normal LD50 toxicity. It doesn't kill by metabolic disruption. It kills by turning the thing from organism into not organism.
If an organism survives it does so not by adapting its metabolism to the poison. It does so by enough of itself not being destroyed that it is able to rebuild.