Feeling congested? Pop some acid, it will clear you right up. Well no. don't really do that. but it is interesting that HCl is the active ingredient in nearly all decongestants, due to the property it has of compressing the blood vessels in your sinuses reducing swelling, and allowing you to both drain, and breathe easier. Now for the math part. One particular fast-acting decongestant contains 15mg HCl and you are supposed to take two every dose. It is known that you only need about 4.5mg in your system to be effective, and that the half life is expected to be about 150 minutes. How long before the dose is no longer expected to be active? What would we expect the box to say about taking more? If we were to continue with what the box says for 5 consecutive doses, how much will be in our system right before we would take the 6th dose?