There is only one effect from asthma, you find it difficult to breath out. This can be caused by reactions to cats, horses or pollen, physical exercise and hyperventilating.
sort of.
Because humans breathe, which means our volume changes significantly with each breath (by a few litres) but our mass stays the same. Since density is mass/volume, the density changes with every breath too.
Difficult to find? No. Difficult to see? Definately.
Breathing underwater can be difficult if you do not have the right equipment. A snorkel can help you to breath or you could use a scuba diving mask with oxygen.
At higher altitudes, the air pressure is lower. Therefore each breath you take contains less oxygen.
It is difficult to breathe on the summit of Mount Everest because there is less air to breathe than what is at sea level.
Nothing to do with bait, of course (as in, the cat ate a lump of cheese and waited outside the mousehole with baited breath). In fact, BATED is short for ABATED, or ceased; it just means holding your breath. Bated breath is breathing that is difficult because of emotion.
As people age and smoke, it is more difficult to breath.
because the air is so thin and there is "less" air
You will find the answer on Wikipedia.
It is a throat disese that is similar to strep throat. It causes your esophagus to close, making it difficult to breath.