LEVELS OF HEAVY METALS IN TRAFFIC- RELATED PARTICULATE MATTER ALONG A MAJOR MOTORWAY IN NORTHEAST LOCAL GOVERNMENT OF IBADAN

Abstract

Traffic-related particulate matter contributes extensively to ambient air pollution especially in urban centres and has been reported to cause respiratory illnesses. Hence, populations found along major urban motorways are at higher risk of various health effects. Although information on traffic-related particulate matter is available in Nigeria, data on particle-bound heavy metals are insufficient. This study was therefore designed to determine levels of heavy metals in the air along IdiApe-Iwo road motorway in Ibadan North-East Local Government Area.

Descriptive study design involving air sampling and particulate analysis for selected heavy metals was used. Five purposively selected sampling locations (SL) 150 metres apart namely: Abayomi Area (AA), Iyana Agbala (IA), Barracks Area (BA), Agbaakin Layout (AL) and Holiness Junction (HJ) were selected along Idi Ape – Iwo road motorway. At each SL, levels of particulate matter (PM10) at intervals of 10, 20 and 30 metres away from the motorway were measured using calibrated fine particulate matter sampler. Also, traffic density was estimated via manual counting. Measurements were done in the morning (7- 10.30 am) and afternoon (1- 4.30 pm) for 10weeks. Filter papers obtained from the sampler from SL were digested and analysed for Lead (Pb), Iron (Fe), Copper (Cu), Cadmium (Cd), Zinc (Zn), Chromium (Cr) and Nickel (Ni) using atomic absorption spectrophotometry according to standard methods. Concentrations of heavy metals were compared with WHO guideline limits. Data were analysed using descriptive statistics, ANOVA and Pearson correlation test at p <0.05.

Daily PM10 level at each SL were AB (100.5 ± 36.3 µg/m3), IA (96.3 ± 42.1 µg/m3), BA (84.6 ± 41.7 µg/m3), AL (110.1 ± 48.4 µg/m3), and HJ (121.6 ± 59.8 µg/m3) was 4-6 times above the PM10 guideline limit of 20 µg/m3. The mean levels of heavy metals were Pb (161.1 ± 108.3 mg/m3), Fe (2.54 ± 1.09 mg/m3), Cu (0.91 ± 0.31 mg/m3), Cd (52.5 ± 30.7 mg/m3), Zn (0.52 ± 0.39 mg/m3), Cr (0.19 ± 0.03 mg/m3) were also found to be higher than the WHO limit for Pb (0.0005 mg/m3), Fe (0.01 mg/m3), Cu (0.01 mg/m3), Cd (0.000005 mg/m3), Zn (0.04 mg/m3) and Cr (0.005 mg/m3). The peak heavy metal concentrations of Cd (248.8 mg/m3), Fe (8.95 mg/m3) were obtained in the morning while Pb (998.6 mg/m3), Cu (0.060mg/m3), Zn (1.03 mg/m3) and Cr (0.42 mg/m3) reached a peak in the afternoon. The mean traffic density was 3644.5± 820.7/hour. Spatial variations in PM10 and heavy metal concentrations were found to reduce with distance further away from traffic by 23.5% and 18.7% respectively. Mean concentrations of the heavy metals were found to increase with a rise in traffic density at each SL. Inter-SL variations were significant for all heavy metals except Pb (F=2.1, p=0.09) and Cu (F=0.18, p= 0.95). A significant positive correlation (r=0.84) was recorded between PM10 and traffic density.

Levels of heavy metals at all sampling locations along Idi-Ape-Iwo road motorway exceeded standard guideline limits for urban centres. Periodic air monitoring of motorways should be instituted and relevant bodies such as Ministry of Environment should formulate policies aimed at reducing traffic-related particulate matter concentrations on our motorways especially exhaust emissions.

Keywords: Air pollution, Heavy metals, Traffic density

Words count: 487