What makes urine smell acrid? Occasionally, after someone's been, there is an awful acrid smell lying about long after them. What causes this? The food that they've eaten? Urea less dilute due to dehydration? If it helps I've noticed it only with older males.

The odor comes from urea. Urea will slowly undergo displacement to become ammonia, but when it's fresh, it has a strong odor. Other odors do come from food, notably coffee and (in some of the population) asparagus.