Lead (Pb) is the most dangerous metal pollutant of automobile exhaust. Lead is a toxic metal that can be harmful to human health and the environment. It is released into the atmosphere primarily through the combustion of leaded gasoline in vehicles. Here are the reasons why lead is considered the most dangerous metal pollutant of automobile exhaust:

1. Toxicity:
Lead is highly toxic and can cause severe health effects, particularly in children and developing fetuses. It can affect the nervous system, leading to behavioral and learning problems, lower IQ, and impaired growth and development. Lead exposure has also been linked to cardiovascular diseases, kidney damage, and reproductive issues.

2. Persistence:
Lead is a persistent pollutant, meaning it does not break down easily in the environment. It can remain in the soil, water, and air for long periods of time, posing a continuous risk of exposure.

3. Bioaccumulation:
Lead can accumulate in living organisms, including plants, animals, and humans, through the food chain. Once absorbed, it can bioaccumulate in tissues and organs, increasing the potential for toxic effects.

4. Widespread Use:
Lead was historically used as an additive in gasoline to improve engine performance. However, its use has been phased out in many countries due to its harmful effects. Despite the ban on leaded gasoline, lead still persists in the environment due to past emissions and the use of lead-based paints, batteries, and other products.

5. Emissions from Vehicles:
Although leaded gasoline is no longer used in most countries, older vehicles and machinery may still emit lead into the atmosphere. Additionally, the combustion of other metals present in automobile exhaust, such as lead-containing lubricants and brake pads, can release lead particles.

6. Airborne Particles:
Lead can be released into the air in the form of fine particles, known as particulate matter. These particles can be inhaled and deposited in the lungs, leading to respiratory issues and cardiovascular problems.

To reduce the health and environmental risks associated with lead pollution, the use of leaded gasoline has been phased out in many countries and replaced with unleaded gasoline. However, it is important to continue monitoring lead levels in the environment and take measures to minimize exposure to this hazardous metal.