Simple meaning of half life?
 

Does half life mean how long a medicine is effective or just how long it takes the body to excrete it?

Half life is typically a measure of how long the drug/substance stays in the body.

https://en.wikipedia.org/wiki/Biological_half-life

Efficacy is another thing entirely.

AMDE studies on performed extensively on pharmaceutical products. https://en.wikipedia.org/wiki/ADME For other substances like heavy metals and VOCs, there will be less data, but you should still be able to find some information.

When you Google, list the substance of interest and pharmacokinetics.

Another thing to Google is the substance of interest and MSDS. Section 3, 11 & 12 will give you all the important that you are looking for - hazards id, toxicology info and ecological info.

Googling that is not for the faint of heart. Pretty scary stuff, especially nuclear medicines.

Technically--
 

--"half life" in pharmaceutical terms is the approximate amount of time (because everyone's metabolism is different) for half of a given dose of a drug to be metabolized and "cleared" by the liver and/or kidneys (depending on the drug--gabapentin, for example, is processed by the kidneys and excreted basically whole, not as metabolites, and pretty much bypassed the liver).

It is more a measure of how long any active therapeutic effect can be thought to last, rather than an actual measure of how effective in symptom reduction a drug dose is.

Half life is not a good measure of therapeutic effects for all drugs.

A good example is methadone. This drug leaves the pain receptors quite fast, but remains in the body longer for elimination. So with its use therapeutically people can get overdosed on it, leaded to respiratory depression, and still be in pain.