Yes, toxins can vary considerably over time, even in just a few days! However, that is to be expected in measurements of environmental contaminants, pollutants, and toxins taken at different time points.
Everyday people are exposed to thousands of chemicals (see below). This may include new exposures and/or potentially ongoing exposures from residential, occupational, recreational, outdoor, industrial settings air, water, and ground contaminants and pollutants. Studies reveal there is even significant variation in toxin levels among people living in shared environments- for more information see: https://pubs.acs.org/doi/10.1021/acs.est.8b01467
Urinary excretion of toxic compounds and metabolites varies significantly both within and between people. A study of within-individual variation in urinary measurement of environmental chemicals found most toxic chemicals varied two-fold to up to ten-fold over a couple of days. The study can be found at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5658679/pdf/jes201654a.pdf
That is why toxicologists recommend serial measurements over time to identify a person's exposures and total toxic burden levels.
