housands of used vehicles are imported into Nigeria each year. Some of these are not supposed to be allowed into the country, having passed the age of serviceability. Many of these vehicles pack up finally after a few years of service on Nigeria roads, Thereby, turning the country into a scrap yard. Worst still, in the absence of appropriate recycling facilities, these vehicles degrade our environment [3]. They pollute the air with harmful exhaust emissions caused by the wear of piston rings, valve seals, valve guides and cylinder bore [4, 5, &7]. The pollutants include: CO, COx, NOx, SOx, Benzene, Chlorinated Organic Compounds, Ozonides and Peroxides. CO 2 has greenhouse effects, NO 2 (oxidizes to HNO 3 ) and SO 2 (oxidizes to H 2 SO 4) , which eventually fall as acid rain or mist or fog.

In Europe, numerous studies have been undertaken by the European Fuel Oxygenates Association (EFOA) to determine the impact of car emissions on human health and the environment. The results were alarming as the findings of EFOA [10] showed that;

? Long-term exposure to air pollution from cars in adults of over 30 years of age caused an extra 21,000 premature deaths per year resulting from respiratory or heart disease. This was more than the total annual deaths of about 9,900 recorded from road traffic accidents,

? Each year, air pollution from cars causes 300,000 extra cases of bronchitis in children, plus 15,000 hospital admissions for heart disease. 395,000 asthma attacks in adults and 162,000 attacks in children.

Author ? : Department of Electronic Engineering, University of Nigeria, Nsukka, Enugu State, Nigeria. E-mail : [email protected]

The 1999 WHO report on health-costs due to road traffic-related air pollution also showed that carrelated pollution kills more people than car accidents in the three European countries where the study took place (Austria, France, and Switzerland).

Ajayi carried out analysis of study to shows the increase in used vehicles imported into Nigeria between the periods of 1988 -2005 using time-series analysis. Although there were reduction of these used vehicles in 1994 and 1998, he however pointed out that the high rate of pollution on the environment was majorly caused by this increased importations [1].

In this study, a practical approach was adopted to investigate the impact of imported used vehicles on the environment by measuring and analyzing certain exhaust pollutants from used vehicles, the measured pollutants include; Nitrogen Oxides, Sulphur Dioxide, Carbon Monoxide, Carbon Dioxide and Unburned Hydrocarbons (NO x , SO 2 , CO, CO 2 , C x H y ).

Measurements on the concentration of harmful exhaust emissions from a number of vehicle engines were taken at some motor parks in both Edo and Delta States of Nigeria. An Exhaust Gas Analyzer with model number "Testo 350 XL" was used to carry out the measurements, by inserting the probe into the vehicle exhaust. The graphical representations and tabular results of measured pollutants from vehicle engines using Premium Motor Spirit (PMS) and Automotive Gas Oil (AGO) are discussed in the next section.

The graphical representations of measured pollutants from vehicle engines using Premium Motor Spirit (PMS) between the ages of 4-20 months are shown in Figures 3(a

The graphs (Figs 3a-Fig 4c) of both Premium Motor Spirit and that of Automotive Gas Oil show a rise in concentration of pollutants released from vehicles older than six months and above, this is known as the 'running-in' period of the engine. At this period the concentration of pollutants are quite low. The second stage, which is known as the 'normal' period in the life of the engine spans from 5-15years. At this stage the pollutant concentration is relatively high; while the sharp rise from 15-25years is the disaster stage of the engine. This last stage needs serious attention, as it is characterized by very high emissions from the exhaust.

It is also observed that the graph of (PMS) for NO x emission shows less concentration when compared to the graph of (AGO) with the same pollutant. The reason could be that most of the vehicles using PMS probably have "Three-Way Catalytic Converters" in their exhaust system, which makes the release of NO x minimal because most of the Nitrogen Oxide has been broken down into Nitrogen and Oxygen by this catalytic converter. This is unlike vehicles of AGO that probably uses the "Two Way Catalytic Converters", whose main function in diesel engines is to reduce hydrocarbon and carbon monoxide emissions.

The results of Table 1 & 2 (in PPM) did not compared favourably with Table 3, as the values of Table 1 & 2 happens to be higher when compared to the Hourly Mean, Daily Average, and Annual Mean values (in PPM) of the National Air Quality Guidelines for Maximum Exposure (EGASPIN) of Table 3. This is a clear indication that most of these vehicles on the Nigerian roads pose a great risk to the environment. The potential harmful effects of these automobile exhaust pollutants on human health and the environment are summarized as follows:

This study has shown that the concentration of pollutants of imported used vehicles within the ages of 5 -25years are much higher than the emission standards set by the "National Air Quality Guidelines for Maximum Exposure" (See Table 3), implying that such vehicles are very harmful to the environment and climate (See Table 4).

This study suggests that the following measures be put in place; ? Used vehicles entering the country must pass an approved emission test to demonstrate that their emission control equipment is functioning as intended, ? Public and consumer awareness campaigns should be created on the havoc of used vehicles on the environment, ? Vehicle inspection centers should be set up to test and certify compliance, ? Vehicle owners should be made to understand why they should regularly go for checks and maintenance, so that exhaust emissions could be reduced.

If these measures are properly observed, greenhouse gases and other harmful substances will be reduced and Nigeria will be making a shift towards a green economy.