CO2 is only a trace gas. And arsenic is "only a trace" element.
"Your children can drink that water, it only contains a trace of arsenic (0.01 ppm is the WHO and US EPA limit)."
This bite is a variaton from another bite, "How can a tiny trace gas change the climate?" (sub. by John Russell) that includes more commentary, and many other examples illustrating that in complex systems, even tiny amounts of highly active substances can can cause large changes.
Another great example of a tiny trace amount having a big effect, from Climate Progress:
"Take 200µg of lysergic acid diethylamide in 200g of non-chlorinated tap water. That's 1ppm of LSD . . . Report back what you've seen."
A short excellent video demonstrating how just 390 ppm of another substance (india ink in water) can block significant amounts of electromagnetic radiation (visible light) can be seen at CO2 Ink Demonstration
Bite source: Skeptical Science, "CO2 is just a trace gas" includes a detailed discussion of this topic.