Nigeria has about 195,000km of road network, including about 60,000km of paved surfaces. The demand for road construction and highway expansion in Nigeria has led to an increase in the number of asphalt production plants in the country.
Hot mix asphalt, used mainly for road construction, is produced by mixing aggregates –- gravel, crushed stones, rock dust or powder – with bitumen under high temperature conditions. This is done using an assembly of mechanical equipment in a plant – a facility which may be a temporary or permanent structure.
These asphalt production plants are sometimes located within residential areas for convenience and proximity to construction sites. Emissions from hot mix asphalt and other plant operations, such as heating of aggregates and bitumen, loading and transportation of materials, as well as vehicle and facility maintenance, introduce various contaminants, like heavy metals, into the environment. It’s important to assess whether these pollutants constitute a public health risk.
Heavy metals are naturally occurring elements that have relatively high density and are toxic or poisonous even at very low concentrations. Examples include mercury, cadmium, copper, arsenic, lead, cobalt, zinc and chromium.
Pollution of the environment by heavy metals has caused some serious public health crises in the past. Minamata disease, for example, caused by mercury poisoning from eating contaminated fish, was documented in Japan in 1953. Itai-itai disease, caused by cadmium poisoning, also occurred in Japan. Nigeria has also experienced its own public health crises as a result of heavy metals pollution. One of these was the lead poisoning incident in Zamfara state as a result of illegal mining.
Aggregates used for asphalt production have varying concentrations of heavy metals. Some of these metals may escape from the aggregates and bitumen during asphalt production into the surrounding soil and water sources.
The metals selected for our study are listed as priority pollutants by the United States Environmental Protection Authority. They have also been found in high concentrations within the environment of industries that employ combustion (burning of fossil fuels) or incineration (burning of waste to ash) as part of their processes. Some have been detected among metals washed off from asphalt pavements.
We studied two hot mix asphalt plants in Rivers State, Nigeria. The plants were in suburbs of Port Harcourt city and had been in operation for 16 years. We collected soil samples from the plants in dry and rainy seasons and analysed them for lead, cadmium, cobalt, manganese, chromium, nickel, zinc and copper. We also assessed the degree of soil pollution by the metals and the health risk consequences of human exposure to these metals.
We did two assessments: pollution assessment, to find out how contaminated the soil is; and health risk assessment, to find out the potential effect of the contamination on human health.
Pollution assessment The concentration of heavy metals in soils from the asphalt plants was within the limit of regulatory standards, except for cadmium. The pollution assessment also revealed the soil samples were highly polluted by cadmium. Cadmium is mostly seen in the environment as a result of the use of fossil fuel, metal ore and waste combustion.
Another recent study, done in Port Harcourt, also showed that soils near asphalt plants were polluted by cadmium and arsenic.
In a separate study, we found that other toxic pollutants such as polycyclic aromatic hydrocarbons were detected in soils within asphalt plant vicinity. They can contaminate nearby water sources, adversely affect biodiversity and can cause cancer.
Health risk assessment We measured the average daily intake of the metals and found that exposure to the soil heavy metals via ingestion and inhalation poses more of a risk than via skin contact. Then, we measured the average daily intake of the metals with lifetime exposure to estimate the cancer risk. We found that the sum of cancer risks from all the exposure routes was within the acceptable limit.
What we recommend
We recommend cleaner and more efficient production of hot mix asphalts to minimise health risks and environmental pollution during both production and paving operations.
Hot mix asphalt plants should not be built within residential areas and around water resources. Workers in hot mix asphalt plants and paving construction operations should use adequate personal protective equipment to minimise exposure to heavy metals and other toxic substances.
We also recommend the gradual transition from asphalt roads to concrete roads which are more environmentally friendly and durable.