Hi M.,

As Terri rightly said, this is not at all a "dumb" question..it is very confusing. 

What you want to look at are the numbers in the SECOND to last column (PLASMA half-life), not the numbers in the LAST column.  Plasma is a component of blood, so PLASMA half-life will tell you how long a substance is expected to stay in blood circulation. 

For example, it is generally well-accepted that a dose of hydrocortisone will last (stay in circulation) for 6-8 hours.  This is based on its plasma half-life of 1-2 hours, also as listed in the second to last column. 

E.g. Say you took 10 mg of hydro.   After 2 hours, half that amount would be left in circulation (5 mg.) After another 2 hours (4 hours total), half of the half would be left (2.5 mg).  After 6 hours, 1.25 mg would be left (half of half of half); and after 8 hours, only 0.625 would be left (half of half of half of half....a neglible amount.) 

If you similarly worked through the math for a dose of dex, using the stated plasma half life of 3-4.5 hours, you will see that a dose is expected to last about 16 hours.