Non-detect means 'below the reporting limit'
Every lab method has a reporting limit: the lowest concentration it can reliably measure. A 'non-detect' (ND) means the contaminant was not found above that limit. It does not certify that the true amount is zero; it may simply be below what the test can see. The reporting limit, not zero, is the reference point.
Why that distinction matters
For a contaminant like PFOA, where the health goal is zero and the federal limit sits near the edge of detectability, an ND is reassuring but not a guarantee. In Tucson Water's EPA record, for instance, PFOA and PFOS were recorded as not detected, below 4 ng/L — meaning below the lab's reporting limit on those sampling dates, not a certified zero. It is why PurityMap shows the reporting context and never dresses an untested or below-limit result up as a clean pass.
Absence of a measurement is not evidence of absence. A contaminant that was never tested is shown as pending, never as a non-detect.
How to use ND results
Treat a non-detect as good news with a caveat: at the time of that sample, the contaminant was below what the lab could measure. Check the date, and remember that monitoring frequency varies. When in doubt for your own tap, a certified home test is the only way to know — see how to test your tap water at home, or open your city water report to view each finding with its date and source.
This article explains how monitoring results are reported. It is not medical advice and does not determine whether any water is fit for a specific person. For health questions about specific contaminants, consult a licensed healthcare professional, and use a certified laboratory to test your own water.