Effect Measure

In our earlier discussion of the science behind greenhouse gases we pointed out that all objects radiate electromagnetic radiation, doing so at a peak wavelength dependent upon their surface temperatures. That means two things. One is that things at the usual temperatures in our world are radiating EM radiation at wavelengths characteristic of the far infrared region. The other is that by measuring the intensity of infrared you can also measure the surface temperature of the body without touching it. Commercial devices are touted as highly accurate. Clinicians use them to measure body “core temperatures” by sticking an infrared sensor in your ear and measuring the infrared (IR) coming off your eardrum. Public health officials have talked about using the same technique at airports or other public places to detect people with fevers as part of trying to stop the spread of infectious diseases. Think of screening passengers remotely (and perhaps covertly) as they embark on a plane. Those with fevers could be pulled aside for a more thorough look. Sounds like a good idea, right? But how good are these devices for this purpose, i.e., detecting a person with a fever by measuring the temperature on some easily visualized body surface like the forehead? A team of French researchers set out to find out.

An IR thermometer (Raynger MX; Raytek, Berlin, Germany) was used to measure forehead temperature on 2026 patients seen in the emergency room of a large Paris teaching hospital and compared to their eardrum (tympanic membrane) temperature, used as a reference or “gold standard.” Here’s what the device looks like in this application:


57% were men and 43% women. Average age was 46 but the age range went from 6 to 103 years old. The average IR skin temp was 36.7°C [“normal” body temperature is 37°C or 98.6°F.], exactly the same as the average tympanic membrane temperature. So far so good. But the same average temperatures doesn’t mean that the skin and tympanic membrane temps matched for individuals. In this case they didn’t:

Correlation between cutaneous and tympanic measurements was poor, and the infrared thermometer underestimated body temperature at low values and overestimated it at high values. Multiple regression analysis showed that 3 variables (tympanic temperature, outdoor temperature, and age) were significantly (p<0.001) and independently correlated with the magnitude of the difference between cutaneous and tympanic measurements. (Hausfater P, Zhao Y, Defrenne S, Bonnet P, Riou B. Cutaneous infrared thermometry for detecting febrile patients. Emerg Infect Dis. 2008 Aug; [Epub ahead of print])

Here’s a scatter plot of the two measurements, the IR on the vertical axis and the tympanic membrane on the horizontal axis. The lack of correlation is pretty dramatic:


The bottom line is that this kind of remote fever sensing had poor positive predictive value, meaning that the proportion of people correctly identified as having fever was low, ranging from 10% to 16%. Thus there were a lot of false positives. Negative predictive value, the proportion of people classified by the IR device as not having fever who in fact did not have fever was high (97% to 99%), so not many people with fevers will be missed with the IR device. Predictive values depend not only on the accuracy of the device but also how prevalent fever is in the screened population. In the early days of a pandemic, fever prevalence will be very low, leading to low positive predictive value. The false positives produced at airport security would make the days of only taking off your shoes look good.

The idea of airport fever screening to keep a pandemic out has a lot of psychological appeal. Unfortunately its benefits are also only psychological: pandemic preparedness theater. There’s no magic bullet for warding off a pandemic. The best way to prepare for a pandemic or any other health threat is to have a robust and resilient public health infrastructure.

I think I’ve said that before. Well I’m saying it again.


  1. #1 anon
    June 25, 2008

    why can’t we just use “tympanic” measurement
    at airports then ?

  2. #2 revere
    June 25, 2008

    anon: Because you have to stick something in someone’s ear, so it’s not remote sensing for quick mass screening. It’s invasive, like doing a body search on everyone. The only point of doing this would be to prevent spread of a flu pandemic and that is a futile exercise.

  3. #3 anon
    June 25, 2008

    you could screen them voluntarily.
    Mesuring fever by whatever method shouldn’t be
    so much “invasive” ?!
    Checking the luggage is more invasive IMO.

    Yes the mass-sensing method could be useful for
    other things but air-passengers “suffered”
    enough from their travel, so fever measuring would only contribute minutely.
    In an epidemic you would do multiple measurements anyway.

  4. #4 non-anon
    June 25, 2008

    The point is that this process would likely overwhelm the system, cause undue panic, not quell the spread of a pandemic, and likely be invasive to the point of violating perceptions of privacy. We’ll probably have to wait until they get infrared screening more valid.

  5. #5 anon
    June 25, 2008

    they give a rather positive impression about fever

  6. #6 CRM-114
    June 25, 2008

    This will isolated the drunks for ‘special screening’.

  7. #7 revere
    June 25, 2008

    anon: As far as I can see, this paper does not address the accuracy of the screening device.

  8. #8 g336
    June 25, 2008

    And the passengers who are identified by these remote sensors as having fevers will be offered homeopathic remedies, right?

    Seems to me that any company promoting a device of that type with a claim of accuracy is engaged in fraud and misrepresentation of the product and ought to be prosecuted unless they can produce supportive findings from an accredited independent testing lab.

    As for privacy invasions, sticking something in your ear pales into insignificance next to having the border guards copy the entire contents of your hard drive. Or having to pull down your pants and give a working demonstration of your “private” parts under the watchful gaze of your employer in order to prove that you’re “pure enough” to work for them. Ears are “public parts”, after all.

    And so long as the inserted device is wearing a disposable plastic protector there’s little if any risk of it becoming a transmission path.

    Though, as Revere says, any such screening is pointless because it won’t pick up asymptomatic carriers.

    Isn’t globalization fun? Anyone want to globalize even more, to have even more fun?

  9. #9 Mark
    June 25, 2008

    I’ve traveled from Hong Kong to mainland China recently and they do this kind of check at the border. Border guards open the door of the car and point what looks like a gun at your forehead. Supposedly, they turn you back if you have a fever.

  10. #10 Red Crayon
    June 25, 2008

    The problem is more the false negatives, IMHO. First
    of all I agree that we should not do this screening
    at airports. OK, so now that that is said, look, if you want to
    reduce false positives, just increase the threshold. Above 39.8
    or so on the y-axis, most of the x-axis readings are above 37.
    False positive problem solved. But then we’ve got a lot of
    false negatives, and from the public health perspective, this
    is the problem… these are the [putative] pandemic spreaders
    who are being allowed to cross international borders. The false
    +ve will be released after 72 hours of “observation” 😉 while
    the false -ve will be released after 0 hours and will spread
    the flu.

    Anyway, as has been said here before, during the prodrome
    one may be an asymptomatic carrier. Plus people have fever
    for so many reasons. Plus, do we want to quarantine the
    person in 16B if the person in 16A is sick?

    Let’s all take a deep breath and remember that civil liberties
    are hard to get back once taken away. The evidence that this
    would do any health good is flimsy. Shocking the recirculated
    air with UV during the flight would probably have a greater net
    reducing effect. And works against bugs with much longer
    latency, too, like tuberculosis.

  11. #11 daedalus2u
    June 25, 2008

    A HEPA filter takes out everything that is alive.

    By the time you screen someone at airport security, how long have they been walking around spreading what ever it is that they have?

    The problem is that these approaches are being thought up by people (such as MRK, but not to pick on him, it is the mindset that most in the Bush administration share) who want to be “tough”, not by people such as Revere who know how to do what is effective.

    Violating people’s civil liberties shows that you are being “tough”, even if it is completely ineffective.

    If your goal is to be effective, you listen to the best science there is. If you goal is to be perceived as “tough”, so you can manipulate the people into doing what you want, then you do “tough” things.

  12. #12 Jon Herington
    June 25, 2008

    Australian epi-modelers did some work on border screening and its effectiveness when our federal government decided to buy $32 million worth of heat-screening gear like the one above.


    Their conclusion – even if you caught 100% of symptomatics – it would give you at most a couple of more weeks of lead time. $32 million dollars that could have gone to better home preparedness or pandemic prevention overseas.

  13. #13 NM
    June 25, 2008

    Public health people proposed this? How embarrasing…

  14. #14 Lea
    June 25, 2008

    Strange to some maybe however, what about the female who is in the middle of a hot flash?

  15. #15 pft
    June 26, 2008

    Having high blood pressure also provides false positives. I used to get stopped regularly in Taipei and Shanghai for an ear measurement. For some reason I always passed HK’s screening. I think it’s mainly for show now, I never get stopped, they really don’t want to be bothered with having to take someone to the hospital and find it it’s regular flu. There are so many reasons for having a fever.

    When a pandemic hits, you will know it. Relax and do not worry about things that can not be controlled.

  16. #16 anon
    June 26, 2008


    seems to me that they only consider pandemics of relatively small damage.
    But at some point of expected damage those costly quarantine methods will pay off.

    see here for the British estimates on the effect of travel restriction:

  17. #17 Tobias Zimmerman
    June 26, 2008

    They have infrared face scanning at the airport in Taipei for people getting of international flights. I always wondered if that was even remotely accurate. Guess not!

  18. #18 Jon Herington
    June 26, 2008

    Anon – I don’t really know what you refer to when you say ‘high damage’ pandemics, so I can only assume that you are referring to high morbility/mortality. As you probably know, those kinds of pandemics are likely to be those that spread more quickly. Caley et al’s modelling shows that as R0 increases, border control measures get less and less effective at delaying the peak. The models you link to also point that out.

    The point is not that the money shouldn’t have been spent – but that it could have been spent in ways that would actually make a difference, like upscaling surge capacity and providing for targeted antiviral pools that can be used to mitigate a pandemic at its source (i.e. in SE Asia). As the modelling points out, spending the money on border screening which has limited utility is a gross misuse of such a large allocation of funds.

  19. #19 anon
    June 27, 2008

    yes, high morbity/mortality, if you prefer.
    I assume that countermeasures increase faster
    than R0.We will do everything to prevent/slow the spread
    of such a pandemic.
    Timing could be essential for vaccine production.
    Weather may help. (seasonality)
    Which modelling says that spending the money on border screening is a gross misuse ?
    Not the British

  20. #20 MoM
    June 27, 2008

    Did I miss the part where people with panflu are infectious BEFORE they become symptomatic (and therefore, febrile)? Works for SARS. For panflu, notsomuch.

  21. #21 anon
    June 28, 2008
  22. #22 Gay Bob
    July 15, 2008

    Everyone knowth that the only way to meathure temperature is to meathure it rectally! Oooo! Me! Take MY temperature! Oh, yeth!

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