1. Molecular Medicine Center, Department of Medical Chemistry and Biochemistry, Medical Faculty, Medical University of Sofia, Sofia, Bulgaria
2. Department of Health Care, UMHAT ‘Medika’, University of Ruse, Ruse, Bulgaria
3. Department of Occupational Diseases, UMHAT ‘Sveti Ivan Rilski’, Medical University of Sofia, Sofia, Bulgaria
4. Department of Public Health and Social Activities, UMHAT ‘Medika’, University of Ruse, Ruse, Bulgaria
5. Department of Pulmonary Diseases, MHATPD ‘Sveta Sofia’, Medical University of Sofia, Sofia, Bulgaria