Hello,

Sorry if this is a stunningly basic question, but humour me...

How is the safety of something worked out from half-lives? That probably
sounds horribly worded, but I was reading something about pesticides
and one comment said:

"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.

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

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

On Sunday, 21 July 2013 23:50:06 UTC+1, wrote:
Hello,



Sorry if this is a stunningly basic question, but humour me...



How is the safety of something worked out from half-lives? That probably

sounds horribly worded, but I was reading something about pesticides

and one comment said:



"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.

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/excrete half the amount of the drug taken. Some drugs may take several weeks to be eliminated to the extent that the residual levels in the body are sufficiently 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.

wrote in message
...
Hello,

Sorry if this is a stunningly basic question, but humour me...

How is the safety of something worked out from half-lives? That probably
sounds horribly worded, but I was reading something about pesticides
and one comment said:

"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,

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

On 22/07/2013 06:18, wrote:
On Sunday, 21 July 2013 23:50:06 UTC+1, wrote:
Hello,



Sorry if this is a stunningly basic question, but humour me...



How is the safety of something worked out from half-lives? That probably

sounds horribly worded, but I was reading something about pesticides

and one comment said:



"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.

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/excrete half the amount of the drug taken. Some drugs may take several weeks to be eliminated to the extent that the residual levels in the body are sufficiently low to avoid a drug interaction.

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

On 21/07/13 23:50, wrote:
Hello,

Sorry if this is a stunningly basic question, but humour me...

How is the safety of something worked out from half-lives? That probably
sounds horribly worded, but I was reading something about pesticides
and one comment said:

"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?
generally ten.

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.

Thanks in advance,

David Paste.


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 22/07/13 06:18, wrote:
On Sunday, 21 July 2013 23:50:06 UTC+1, wrote:
Hello,



Sorry if this is a stunningly basic question, but humour me...



How is the safety of something worked out from half-lives? That probably

sounds horribly worded, but I was reading something about pesticides

and one comment said:



"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.
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/excrete half the amount of the drug taken. Some drugs may take several weeks to be eliminated to the extent that the residual levels in the body are sufficiently 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.
No, thats not the meaning of half life he this about how long it
takes to break down in the environment.


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

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

--
From the Sofa of Brian Gaff Reply address is active
wrote in message
...
Hello,

Sorry if this is a stunningly basic question, but humour me...

How is the safety of something worked out from half-lives? That probably
sounds horribly worded, but I was reading something about pesticides
and one comment said:

"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.

"harryagain" wrote in message
...

wrote in message
...
Hello,

Sorry if this is a stunningly basic question, but humour me...

How is the safety of something worked out from half-lives? That probably
sounds horribly worded, but I was reading something about pesticides
and one comment said:

"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,

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

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

On 22/07/2013 09:15, tim..... wrote:

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

Tell that to the homeopathists.

On Sun, 21 Jul 2013 22:18:38 -0700 (PDT), wrote:

Two different meanings of half-life here.

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.

--
Cheers
Dave.

"Brian Gaff" wrote in message
...
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.

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.

On Monday, July 22, 2013 12:33:12 PM UTC+1, Mentalguy2k8 wrote:
"Brian Gaff" wrote in message

...

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.



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

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...

On 22/07/2013 00:05, Tim Streater wrote:


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.


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

--
Reentrant

On 22/07/2013 14:15, Reentrant wrote:
On 22/07/2013 00:05, Tim Streater wrote:


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.


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

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

On 22/07/2013 09:04, Brian Gaff wrote:
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

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

On 22/07/2013 06:18, wrote:
On Sunday, 21 July 2013 23:50:06 UTC+1, wrote:
Hello,



Sorry if this is a stunningly basic question, but humour me...



How is the safety of something worked out from half-lives? That probably

sounds horribly worded, but I was reading something about pesticides

and one comment said:



"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.

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/excrete half the amount of the drug taken. Some drugs may take several weeks to be eliminated to the extent that the residual levels in the body are sufficiently 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.



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.....

tim..... wrote:
"harryagain" wrote in message
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

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

Sort of like the Ford Orion.

--
Adam

GB wrote:
On 22/07/2013 09:15, tim..... wrote:

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

Tell that to the homeopathists.

:-) Thank you

--
Adam

"Stephen H" wrote in message ...

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.....

LD50 is an incredibly crude approximation. I once conducted a series of
experiments in which we measured the LD50 to be approximately 10 times
higher than the previously published data. Admittedly the previous data was
40 years old by that point and a lot of experimental techniques (and for
that matter animal strains) had changed in that time. That is not to say
that LD50 is useless - it works well as a 'quick & dirty' approach when
setting up pilot studies but otherwise it is only really useful as a
comparative measure of toxicity.

Cheers

Mark

"RobertL" wrote in message
...
On Monday, July 22, 2013 12:33:12 PM UTC+1, Mentalguy2k8 wrote:
"Brian Gaff" wrote in message

...

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.



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

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...

Wasn't that one of Aesop's fables?

Thanks all for the replies, I had a vague idea what the answer was, but I'd rather ask than potentially get it wrong.

Cheers again.