Yeah, 0.6 ppm may sound like a very small amount, but I thought the problem is that the effect over time / harm as a function of concentration levels is convex;

Yeah, 0.6 ppm may sound like a very small amount, but I thought the problem is that the effect over time / harm as a function of concentration levels is convex; i.e. +0.6ppm today means a huge difference 10 years from now.