Cycling can be an excellent cardio workout if you work at a high enough intensity. It’s generally easier to elevate your heart rate by running, climbing stairs, or using an elliptical trainer with moving arms, because these involve more muscle groups. The more muscles you move, the higher caloric burn you’ll experience. And if you’re riding outside, you’re likely coast as well as pedal; you’ll have to stop at intersections and for pedestrians, so your heart rate will be hard to maintain. So it may not be the best to boost your cardiovascular health but it’s still better than nothing!!