Californians love to be outside. But even on a clear, sunny day, we know that the air we are breathing isn’t as clean and clear as it looks, especially in our increasingly congested and crowded cities. Air quality is a complex topic that is affected by the weather, the time of day, and events like wildfires. But one aspect of air quality is very clear: tailpipe emissions make air quality worse. When those emissions happen in cities, millions of people are exposed to harmful pollution.