Like on Mammoth Mountain, it kills plants when toxic levels are reached. Few studies exist, but one put the value at 10,000 ppm for maize and other crops.
Most hydroponic growers say the limit is 2,000 ppm, and they will rarely target beyond 1,500 ppm because it stresses the plants too much.
600 to 1,000 ppm is quite common for indoors living spaces. Outdoors, roads will often reach 750 ppm in heavy traffic and cities can reach 1,000 ppm periodically.
My building has very poor ventilation and I am sure I regularly exceed 5,000 ppm in some rooms. With indoors livestock farming it is a major issue.
There might not be any need to worry about enrichment if you have high ambient levels.