The phrase "low levels of chemicals" refers to a situation where there is an insignificant or minimal amount of chemical substances present. Full definition
This continuing focus may be attributed, in part, to our increased ability to detect low levels of chemicals in the environment. (acsh.org)
I would like to point out that second babies and beyond get lower levels of the chemicals as do babies who breastfeed for extended periods of time... as the chemical load of the mothers is decreased if they watch what they ingest. (ecochildsplay.com)
With a compromised immune system, the affect of even low levels of chemical exposure is enhanced (whereas another person with a strong immune system exposed to same chemicals might not be compromised in the same way). (thetot.com)