(somewhat) O/T: Question about half-lives.

Hello,

Sorry if this is a stunn "Half life of Imidacloprid-1-4 hours....so by the time I go to eat the food all of it is gone.

Thiamethoxam half life=4 days."

So, how do you work out how many half-lives until something is safe?

Thanks in advance,

David Paste.

Reply to
pastedavid
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After one half life, there's half left. After two, there's a quarter left, after three, an eighth, and so on. The question of when it's safe, whatever it is, depends then on the initial dose or quantity, and how much is a dangerous level. Also how much is ingested and then goes somewhere where it matters.

F'rinstance, Polonium 210, which did for that Russian, has a half life of 138 days. But I could carry a bottle of say polonium nitrate around in my pocket, no problem, because the radiation given off won't even penetrate a sheet of paper. But drink it, and because it looks to the body like calcium, off it goes to the bone marrow, where the radiation is able to kill your bone marrow cells, and then you.

But wait 5 years and there'll be only 1/8000 of it left in the bottle. You might then get away with drinking it. That would depend on how much you drank.

So the answer is, it depends on a number of factors, of which half life is one.

Reply to
Tim Streater

The starting concentration and the poisonous concentration.

If you start with 1000 units and anything over 100 units is poisonous then 3.5 half lives will get you down to 100 units (three halvings of 1000 is 125).

If you start with 1000 units and anything over 10 units is poisonous then 7 half lives will get you down to 10 units (seven halvings of 1000 is 7.5somthing)

The easy thing to remember is as 2^10 is 1024, ten half lives will get you to less than 1/10% of the original quantity.

JGH

Reply to
jgh

Two different meanings of half-life here.

In the radioactive element sense, the half-life is the time taken for half the amount of that element to decay into other elements in its decay series . Varies from the tiniest fraction of a second, to thousands of year.

In the pharmaceutical sense, it's the time for the human body to digest/exc rete half the amount of the drug taken. Some drugs may take several weeks t o be eliminated to the extent that the residual levels in the body are suff iciently low to avoid a drug interaction.

I'm guessing that the latter sense is also applied to cumulative poisons in the human body. Remember several half-lives may be necessary to eliminate enough of the substance for the residual levels to be inconsequential.

Reply to
dom

It is a way or measuring a rate of change and is applied to some processes that are not linear. In practice the effect/change continues forever but getting smaller and smaller.

In the case you mention, the stuff never completely disappears. Half remains in 4 days Quarter remains in 8 days one eigth remains in 12 days. One sixteenth remains in 16 days One thirtysecond remainds in 20 days And so forth

Reply to
harryagain

This is a follow-up question to the insecticide (Raid) one. The half-life is referring to the danger of the active ingedient after it may have been sprayed on food, but before being eaten. So the radioactive half-life is the more similar analogy.

SteveW

Reply to
SteveW

But the reality is that there is a lower limit of concentration below which something is considered 'safe' and that depends on the original concentration dived by two to the power of the number of half lives.

Reply to
The Natural Philosopher

No, thats not the meaning of half life here: this about how long it takes to break down in the environment.

Reply to
The Natural Philosopher

Yes I've often wondered this. After all if its half the amount, the there is never going to be nothing there. Its the old joke about every day you get half as close but never actually arrive. Brian

Reply to
Brian Gaff

Though we are working with integer mathematics

You will eventually get to a point where the number of molecules remaining is 1 and that will decay to zero

tim

Reply to
tim.....

Tell that to the homeopathists. :)

Reply to
GB

A half life, is a half life. ie. how long it takes for half of the orginal substance to have "disappeared". However the enviroment that that substance is in does affect how long that period is.

In the case of tetramethrin (one of the active ingredients in fly spray) the half life in soil is 30 mins, in air 4 hours or there abouts.

There is no easy answer to the OPs question as it depends on how much you have, how nasty the orginal substance is and the enviroment. Suffice to say that after 10 times the half life you have only about

1/1000th of the orginal material left.
Reply to
Dave Liquorice

I think there's a point where it decays enough to become insignificant to human health, and then reaches the further point where it's so insignificant as to be regarded as "nothing".

It reminds me of the old scenario (in reverse) about putting one grain of sand on a chess square, 2 on the next, then 4, 8, 16, 32 etc etc and you end up with 18,446,744,073,709,551,615 grains of sand on the board. Which is rather a lot and totally unfeasible, but just shows how the doubling or halving of something can reach either stupidly huge or relatively insignificant proportions pretty quickly.

Reply to
Mentalguy2k8

Was it not a grain of rice rather than sand? The inventor of chess was asked by the rajah what reward he would like. He asked for one grain of rice on the first square, 2 on the next, 4 on the next and so on. the rajah foolishly agreed...

Reply to
RobertL

Another factor is the decay product. If a radioactive ultimately decays to a stable Pb isotope, you could die from conventional lead-poisoning.

Reply to
Reentrant

Not in the case of Polonium. IIRC the lethal ingested dose is around 1 microgram, but a microgram of lead is nothing.

Reply to
newshound

Well actually, since everything is made from atoms and molecules, you do arrive at nothing eventually.

Reply to
newshound

There is also the LD50 which is the amount (in mass) required to kill

50% of a specific population. The LD refers to lethal dose.....
Reply to
Stephen H

Sort of like the Ford Orion.

Reply to
ARW

:-) Thank you

Reply to
ARW

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