A paper from China presents data on eight nitrogen compounds in human urine (301 samples from 13 Chinese cities): cyanuric acid, melamine, ammelide, ammeline, benzoguanamine, 2,4,6-trimethoxy-1,3,5-triazine, 2,4-diamino-6-(4-methylphenyl) – 1,3,5-triazine, diphenyl amidophosphate. Only first four were widely found in urine. pinfa notes however that only a very small proportion of the widespread use of melamine in consumer and industrial products is for fire safety; that some melamine compounds are used as flame retardants but that quantities are very small compared to the many uses of melamine in different applications. pinfa notes that FRs are < 5% of total melamine use, mainly as an intermediate for melamine compounds, several of which qualify for Ecolabels (see pinfa Newsletter n°129). There is no reason to suggest that flame retardants make any significant contribution to the presence of these nitrogen compounds in urine.
See also Shi et al. 2020
“Exposure to nitrogenous based flame retardants in Chinese population: Evidence from a national-scale study”, Y. Shi et al., J. Hazardous Materials 445 (2023) 130653 https://doi.org/10.1016/j.jhazmat.2022.130653