Exposure to ultraviolet...

[Externet]

For a particular germ, there may be a particular ultraviolet wavelength that is more effective for disabling/killing it.  What is known about exposure times ?  Is it in the seconds, minutes, hours ?

If using a  wavelength that is not optimal for a particular germ; what is known about effectiveness by lengthening exposure time or increasing intensity ?

 

Edited.  found 10 seconds to be  effective.  Surprisingly short time.  But does not specify when wavelenght is shifted.

"The exposure of germicidal ultraviolet is the product of time and intensity. High intensities for a short period and low intensities for a long period are fundamentally equal in lethal action on bacteria. The inverse square law applies to germicidal ultraviolet as it does to light: the killing power decreases as the distance from the lamps increases. The average bacterium will be killed in ten seconds at a distance of six inches from the lamp.    Ultraviolet light in the germicidal wavelength - 185-254 nanometers - renders the organisms sterile. When organisms can no longer reproduce, they die."

Edited May 8, 2018 by Externet
Added text

[swansont]

If the effect is from ionizing, (and in this case it likely is) then shifting the wavelength to a shorter wavelength won't matter, since you are still ionizing whatever molecule is in question. It's not a resonance phenomenon. Going to a longer wavelength could have an effect, as you might no longer be ionizing anything.

 

[PhilGeis]

Bacterial kill and oxidative effect are reportedly wavelength, intensity and exposure dependent.  UV-C (~254 nm) is most cited for germicidal treatment ( https://www.cdc.gov/niosh/docs/2009-105/pdfs/2009-105.pdf,  https://link.springer.com/article/10.1007/s00203-012-0847-5).  

 

[swansont]

I suspect 254nm is reported because it is a mercury line, not because of any special effectiveness it has.

As the cdc links notes, 

"Other UV lamps are designed to emit radiation at 184.9 nm and produce ozone, which is hazardous to humans even at low concentrations"

So that doesn't mean the wavelength less effective, it's just that there's another effect to be considered.  

[Externet]

Thank you, gentlemen.

From the link in post #3, can you please check if am translating properly " Survival and activity followed a clear wavelength dependence, being highest under UVA and lowest under UVC. "?

As highest survival of germs on UVA (400-320 nm) and most damaging/impairing/lethal on UVC (290-100 nm) ?

[PhilGeis]

That other wavelengths produce ozone at some level does not mean kill is wavelength independent.  Suggest you folks read the CDC doc and cited article.  efficacy is wavelength, intensity and exposure dependent.

 

 

[PhilGeis]

Thymidine dimer (in DNA) formation is one of the primary mechanisms of UV damage - cidal at microbial cellular level and  a major cause of skin cancer.  This is wavelength dependent as well.http://www.pnas.org/content/pnas/86/14/5605.full.pdf

[swansont]

1 hour ago, PhilGeis said:

That other wavelengths produce ozone at some level does not mean kill is wavelength independent.  Suggest you folks read the CDC doc and cited article.  efficacy is wavelength, intensity and exposure dependent.

That's from the abstract; the full article is paywalled. I want to see the paper. It may very well be that the same power level gives different efficacy. A photon at 150 nm is twice as energetic as one at 300 nm. So at the same power, you have fewer photons. If the efficacy scales with that, then it means that they are the same on a per-photon basis, and the real effect is turning down the flux.

But without access to the paper, one can't tell.  

[John Cuthber]

The 184.9 nm light is, at best, poorly transmitted by air.

The oxygen absorbs it.

 

At least some bacteria (and other microorganisms) generate coloured materials to act as a "sunshade". These will generally be harder to kill

Also, if the bacteria are in, for example, mud, they are pretty safe.

[PhilGeis]

True John - can really see it with some fungi ,melanin of which protects vs. all ionizing radiation.  Some folks suggest it even allows use of radiation as energy source.  Microbes also have dark and light (photoreactivation) repair of UV damage.  

Swansott -CDC cite is a link, not an abstract.  For the articles, why don;'t you contact the authors?  Folks often send papers for discussion - esp  if you tell'em they're wrong.

Edited May 9, 2018 by PhilGeis

[swansont]

3 hours ago, PhilGeis said:

 Swansott -CDC cite is a link, not an abstract. 

The quote you offered up about "Survival and activity" is not from the CDC link (or, at least, Adobe offered up zero matches for a search on that phrase)

3 hours ago, PhilGeis said:

For the articles, why don;'t you contact the authors?  Folks often send papers for discussion - esp  if you tell'em they're wrong.

I wasn't the one who offered it up as an answer. I didn't say they were wrong, I said I needed more information. You, on the other hand, seem to be asserting that you are right, and how can you do that without having read the paper? And if you have, why can't you quote the relevant parts to address my inquiry, instead of passing the buck?

[PhilGeis]

I'm a microbiologist familiar with the subject but offered no opinion - only info from and linkage to relevant published reports to inform your unreferenced opinion.  You may reject PNAS, etc. as you wish.