if that's primarily starting in the lower leaves and there's no ozone generator near the plant, I'd have to guess Mg deficiency.
If its magnesium deficient than give it some Epsom salt and it will clear up. However, I am not sure that is what your problem is. Please give us some more info such as: What type of lights and how far from plants what type of soil is it and are there any added nutes What has the fertilizing schedule been Have the plants been around an ozone filter How old and what size are the plants Any other important info