"Trace levels" refers to extremely small amounts or concentrations of something. It implies that the substance or element being discussed is present only in very minute quantities, often undetectable or barely detectable. Full definition
Although studies have failed to establish a causal relationship, some scientists and activist groups continue to emphasize the role of trace levels of synthetic chemicals in human illness. (acsh.org)
While these two foods don't necessary meet the criteria of the dirty dozen, they contain trace levels of highly toxic pesticides. (ericamesirov.com)
However, even when we are healthy, our bodies carry trace levels of these proteins — known as «inflammatory markers» — which rise exponentially in response to infection. (sciencedaily.com)