You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Biomarkers of Lead Exposure Among a Population Under Environmental Stress. This study aimed to evaluate the relationship between blood lead and serum creatinine and blood lead and serum urea nitrogen levels as biomarkers of lead exposure from subjects living in a historic polymetallic mining area in China. Elevated levels were found for blood lead, serum creatinine, and serum urea nitrogen in the mining area with mean values at 245.65 μg/l, 74.16 μmol/l, and 12.79 mmol/l, which were significantly higher than those in the control area, respectively. Moreover, the coefficients between paired results for blood lead and serum creatinine and blood lead and serum urea nitrogen were positively statistically significant (serum creatinine vs. blood lead, r = 0.35, p < 0.05; serum urea nitrogen vs. blood lead, r = 0.48, p < 0.05). With respect to the effects of sex and age on the blood lead, serum creatinine, and serum urea nitrogen levels, data analysis revealed there was a tendency for higher blood lead, serum creatinine, and serum urea nitrogen levels in females than in males, and the levels of blood lead, serum creatinine, and serum urea nitrogen increased among older residents. We conclude that females and the older population in the mining area are more susceptible to lead exposure. Blood lead, serum creatinine, and serum urea nitrogen can be useful biomarkers of lead exposure among populations under environmental stress.
{"Chemical": ["nitrogen", "creatinine", "urea"]}