It is now well established that air pollution in Mega cities is an important cause of excess mortality. The metropolitan area of Port au Prince, with 2.6 million inhabitants is one of the biggest Caribbean conurbations. In the capital of Haïti, people and goods transport are exclusively provided by motor transportation on road. Moreover, the traffic of Port au Prince is concentrated on ten mean roads before it was spread on secondary urban roads. In April 2019, we counted the number and types of vehicles (cars, heavy vehicles, Bus and Minibus) which use one of the main streets of the city. The mean power, fuel type and age of the fleet, have been deduced from data provided by the Office of Vehicle Insurance Third-Cons (OAVCT) and from European and American standards issue. Those data have been analyzed in order to obtained statistic parameters such as car, bus, and lorry mean polluted gas production. The pollution emitted from this road was calculated using the count and the statistic elements. A previous emission inventory has been done seven years before on the same road with the same counting technique. The traffic, the motor technology and the type of vehicle evolution with the increase of the population. A link between the 13% inhabitant increase ratio and the pollution product behavior of Port au Prince metropolitan area, has been found.