Introduction & Objective: Benzene is a light yellow liquid with aromatic odor and has effects to human body. The main and dangerous health effect of chronic exposure to benzene in workplace is hematopoetic system disease or blood cancer that it^'s primarily clinical figures are anemia, leucopenia, thrombocytopenia. The objective of this study was evaluation of benzene exposure by analysis of urinary trans, trans-muconic acid (t,t-ma) in post shift of workers.

Materials & Methods: A case-control study was conducted. The urine samples were collected from 42 workers exposed to benzene at a cokery plant and it’s by –product refinery site and 40 non exposed villagers from rural areas without nearby factories. The t,t-ma was extracted from urine by solid phase extraction (SPE) and analyzed via high performance liquid chromatography (HPLC). The urinary creatinine level was measured by Kone – Pro autoanalyzer .

Results: The mean urinary t,t – ma level was 3.33 mg/g creatinine for exposed workers and 0.1007 mg/g creatinine for non exposed workers. The urinary levels in exposed group  differed among workplaces, and were higher for subjects that worked in cokery plant ( cokery plant, 3.68 mg / g creatinine its by product refinery site, 0.64 mg / g creatinine).

Conclusion: The results showed that the mean level of urinary t,t-ma was about 6 to 7 times more than biological exposure index thus, there is a high risk of hematopoetic damage and other adverse effects in these workers. We suggest that the company must decrease benzene exposure via engineering and management controls to lower than threshold limit value.