Source: Bitter veteran
Second, the copper jacket on rounds like the cheap stuff you train with tend not cover the back of the round leaving exposed lead - so a lot of lead fouling gets generated by the hot gases atomizing the lead on the tail of the bullet. That makes it into the air, ground, and surroundings when you fire. So you breath some in before the range ventilation ever has a chance. I feel that's probably especially bad at indoor ranges because you've got partitions on either side of you. lead's also great at sticking to shoes, clothes, etc and going home with you. So - de-leading soap ftw!
Some rifles make this worst (especially if shooting suppressed or if you're a lefty) because the gas is much closer and much more in your face).
Most indoor ranges also have subpar ventilation. So if shoot at the indoor range i try to limit my time, ask to be on a range with less other active shooters, use fully enclosed metal jacketed round's, use de-leading wipes when I'm done, and don't bring my shoes into the house when I get home. Really, I just stick to outdoor ranges where I can snag a big private bay. Even then, I still use de-leading wipes on all the things.
That costs extra.