High carbon monoxide (CO) concentrations can elicit adverse health effects. We assessed CO concentrations at filling stations and determined carboxyhemoglobin (%COHb) levels and health problems reported by filling station attendants (FSAs) via questionnaires. This cross-sectional design studied 20 filling stations from Ibadan North Local Government, Nigeria. Outdoor CO concentrations (ppm) were measured for 8 weeks in August–September 2015 from 8:00–10:00 a.m. and 12:00–2:00 p.m., and %COHb levels were measured among 100 FSAs. Data collected were analyzed using Student’s t-test and analysis of variation (p = .05) and compared with relevant guideline limits. Mean CO concentrations in morning (15.4 ± 2.1 ppm) and afternoon (11.6 ± 1.4 ppm) were higher (p < .01) than the World Health Organization (WHO) guideline of 9.0 ppm. Mean %COHb for FSAs (11.1 ± 2.6) was significantly higher (p < .01) than the WHO guideline of 2.5%. Among respondents, 13.4% of FSAs vomited and 14.9% of FSAs experienced nausea. FSAs need personal protective equipment and filling stations should modernize pump delivery systems to minimize exposures.