Neglected Heavy Metals in the Niger Delta Region of Nigeria: A Systematic Review of Toxicity, Environmental Contamination, Health Risks, and Research Gaps
Ibioku Elekima *
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
Eno Richard Okon
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
Queen Jacob Nwazim
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
Ruth Clinton Esele
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
Juliet Nnadi
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
Baribor Lakpege
Department of Clinical Chemistry, Rivers State University, Port Harcourt, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Background: The Niger Delta region of Nigeria hosts one of the largest crude oil reserves in Africa. Decades of oil exploration and production have released a wide range of toxic heavy metals into the environment. While lead (Pb), cadmium (Cd), and nickel (Ni) have attracted considerable scientific attention, other metals, including vanadium (V), arsenic (As), barium (Ba), cobalt (Co), and manganese (Mn), remain underreported despite having well-established toxic effects in humans. This review refers to these metals collectively as neglected heavy metals.
Objective: To systematically review published evidence on the occurrence, distribution, sources, exposure pathways, and human health risks of neglected heavy metals in the Niger Delta region of Nigeria, and to identify existing research and policy gaps.
Methods: A systematic literature search was conducted across PubMed, Scopus, Google Scholar, Cochrane Library, BIOSIS, Global Health, and ScienceDirect, covering publications from 2010 to 2026. Studies reporting measured concentrations of vanadium, arsenic, barium, cobalt, or manganese in any environmental or biological matrix from the Niger Delta were eligible. Screening, data extraction, and quality assessment followed PRISMA 2020 guidelines. Thirty-eight studies met the final inclusion criteria. The initial search for neglected heavy metals in the Niger Delta identified 4,019 unique publications from Google Scholar, PubMed, Cochrane Library, BIOSIS, and Global Health. Of the 4,019 articles, 3,453 were excluded due to duplications while 513 were excluded because of lack of defined location, studies conducted outside the Niger Delta, studies focusing on mercury, lead, and chromium, among others, and studies not reported in the English language. A total of 52 full-text articles were reviewed for eligibility of which 14 records were excluded because they were systematic or mini reviews. Overall, 38 full articles were included in the study.
Results: All five neglected heavy metals were consistently detected across water, soil, sediment, seafood, food crops, and biological samples in the Niger Delta. Concentrations frequently exceeded WHO, US EPA, and Nigerian DPR regulatory thresholds. Arsenic and manganese were the most widely reported. Cobalt showed the highest pooled concentration in seafood at 4.039 mg/kg dry weight. Vanadium was confirmed as a neuropathological hazard in ecological studies. Barium was primarily linked to drilling fluid discharge. Human health risks include neurotoxicity, carcinogenicity, kidney and liver damage, and reproductive harm.
Conclusion: Neglected heavy metals represent a serious but poorly characterised public health threat in the Niger Delta. Comprehensive biomonitoring studies, stronger regulatory frameworks, and community-level health surveillance programmes are urgently needed. Scientific and policy attention must expand beyond conventionally studied metals to capture the full toxic burden facing Niger Delta communities.
Keywords: Neglected heavy metals, Niger Delta, vanadium, arsenic, barium, cobalt, manganese, crude oil pollution, environmental contamination, biomonitoring, health risk assessment