**Primary Gaseous Pollutants in Vehicular Emissions**

- Vehicular emissions are a significant source of air pollution, releasing a variety of pollutants into the atmosphere. These pollutants can have adverse effects on human health and the environment.

- The most significant primary gaseous pollutant found in vehicular emissions is carbon monoxide (CO).

**Explanation:**

- Carbon Monoxide (CO) is a colorless, odorless, and toxic gas that is produced by the incomplete combustion of fossil fuels, such as gasoline or diesel, in vehicles.

- When fuel is burned in an engine, carbon monoxide is produced due to insufficient oxygen supply for complete combustion. This incomplete combustion occurs due to various factors like inefficient engine design, poor fuel-air mixture, or malfunctioning emission control systems.

- Carbon monoxide is particularly harmful because it can bind to hemoglobin in the blood, reducing its ability to carry oxygen to vital organs and tissues. This can lead to various health issues, including headaches, dizziness, fatigue, and even death in severe cases.

- In addition to its direct health effects, carbon monoxide also contributes to the formation of ground-level ozone (O3), a secondary pollutant. Ozone is a harmful gas that can cause respiratory problems, eye irritation, and other health issues.

- Although carbon dioxide (CO2) is often mentioned in discussions about vehicle emissions, it is not considered a primary pollutant. CO2 is a greenhouse gas that contributes to climate change but does not have direct adverse effects on human health.

- Sulfur dioxide (SO2) is another common air pollutant, but it is not primarily associated with vehicular emissions. SO2 is primarily released from the combustion of fossil fuels in power plants and industrial processes.

- Therefore, in the context of vehicular emissions, carbon monoxide (CO) is the most significant primary gaseous pollutant. Its toxic nature and adverse health effects make it a major concern for air quality and public health.