Your lung capacity is a 2-part system.
a) Actual air capacity. How much air can you breathe in/out in one deep breath
b) Useable capacity. How long can you hold your breath before your muscles feel weak and you have to take another one?
One exercise to increase capacity (which is best done in private to avoid embarassment)... While sitting, put your arms straight up over your head, then take as deep of a breath as you can. Slowly lower your arms back to your side and continue to hold your breath. Count off 10 seconds, slowly breathe back out. Take a few good breaths, then repeat for 5 minutes.
Do this whenever you have a spare 5 minutes with nothing better to do.
You won't notice instant results, but after a few days, start counting off 15 seconds, then 20, never go more than 30.
This process is basically a healthy stretch for your lungs. It is impossible to burst or tear your lungs with normal breathing, but expanding the air pockets provides more room for more air.