Do cool white LEDs emit UV light?
Is there any truth in this claim?


Truth, No. 

It is possible there is a tiny bit of UVA and UVB. Amounts so small they would be very difficult to measure. 

Occasionally there are some papers that report a couple of spikes of UV from the yellow phosphors used to convert blue to white. The reason they are reported is they are hoping to find phosphors that convert down to UV wavelengths and when they find a hint of UV it will likely be reported.  Sample of such a paper.  I do not know any of these phosphors that have been reported are used in commercial lighting.

I have some measurements from a horticulture research project at the University of Florida. I used a StellarNet BLUE-Wave Spectrometer, 


200-1150nm wavelength
Signal to noise = 1000:1


a decent $15,000 spectrometer, to measure the Photon Radiance of one of my LED grow lights.  This fixture uses the standard color LEDs which includes the deep blue (Cree XP-E) that white LEDs are made from.

I was able to measure a well formed peak as low as 310nm, two wavelengths adjacent at 288 and 289nm and one spike at 280nm.  

The UVA and UVB wavelengths were very low reading, but well above the noise level. So there are probably some UV emitting from a cool white LED, but they cannot be read with my spectrometer.

I do not use "low quality" LEDs, only Cree and Phillips Luxeon.

The day before I had measured a fixture with Luxeon LXML-PWC2 4000K LEDs. Not a cool blue.  The lowest measurable wavelength for the 4000K was at 409nm @ 0.00077778. For comparison the peak at 310nm on the graph below, was 0.0034645 (4.45x larger)  

These are the readings I got from the spectrometer from 280 to 400nm.
first column is wavelength, second is relative to micromoles/m2/s. By relative, I mean, I forgot what aperture was used. Readings were taken October 7, 2016 in a grow tent at my house.  The current flowing through the LEDs was 350 mA.

 280.00  1.1290E-002
 281.00  0.0000E+000
 282.00  0.0000E+000
 283.00  0.0000E+000
 284.00  0.0000E+000
 285.00  0.0000E+000
 286.00  0.0000E+000
 287.00  0.0000E+000
 288.00  1.1274E-002
 289.00  9.0670E-003
 290.00  0.0000E+000
 291.00  0.0000E+000
 292.00  0.0000E+000
 293.00  0.0000E+000
 294.00  0.0000E+000
 295.00  0.0000E+000
 296.00  0.0000E+000
 297.00  0.0000E+000
 298.00  0.0000E+000
 299.00  0.0000E+000
 300.00  0.0000E+000
 301.00  0.0000E+000
 302.00  0.0000E+000
 303.00  0.0000E+000
 304.00  0.0000E+000
 305.00  0.0000E+000
 306.00  0.0000E+000
 307.00  1.4569E-002
 308.00  1.4059E-002
 309.00  2.4614E-002
 310.00  3.4645E-002
 311.00  2.5260E-002
 312.00  6.2953E-003
 313.00  3.6099E-003
 314.00  0.0000E+000
 315.00  0.0000E+000
 316.00  0.0000E+000
 317.00  7.2641E-003
 318.00  1.4983E-002
 319.00  9.9858E-003
 320.00  3.7277E-003
 321.00  5.7936E-003
 322.00  4.5364E-003
 323.00  2.1041E-003
 324.00  1.3479E-002
 325.00  9.7610E-003
 326.00  2.4578E-003
 327.00  3.9432E-003
 328.00  7.8247E-003
 329.00  3.6119E-005
 330.00  0.0000E+000
 331.00  0.0000E+000
 332.00  0.0000E+000
 333.00  5.3355E-003
 334.00  6.3256E-003
 335.00  1.6233E-003
 336.00  9.1491E-003
 337.00  5.3881E-003
 338.00  7.1098E-004
 339.00  3.9042E-004
 340.00  9.4603E-004
 341.00  0.0000E+000
 342.00  5.8628E-003
 343.00  1.9500E-003
 344.00  0.0000E+000
 345.00  0.0000E+000
 346.00  0.0000E+000
 347.00  1.8100E-003
 348.00  8.4942E-003
 349.00  1.1109E-003
 350.00  0.0000E+000
 351.00  0.0000E+000
 352.00  0.0000E+000
 353.00  0.0000E+000
 354.00  0.0000E+000
 355.00  7.3287E-004
 356.00  6.0877E-003
 357.00  1.8842E-003
 358.00  0.0000E+000
 359.00  0.0000E+000
 360.00  0.0000E+000
 361.00  0.0000E+000
 362.00  2.4273E-003
 363.00  4.5285E-003
 364.00  3.7282E-003
 365.00  1.7190E-003
 366.00  4.4021E-003
 367.00  1.1359E-002
 368.00  1.2333E-002
 369.00  1.9218E-003
 370.00  0.0000E+000
 371.00  1.8477E-003
 372.00  5.1049E-003
 373.00  8.0459E-003
 374.00  1.9174E-003
 375.00  0.0000E+000
 376.00  0.0000E+000
 377.00  0.0000E+000
 378.00  0.0000E+000
 379.00  1.3191E-003
 380.00  8.7230E-003
 381.00  6.5227E-003
 382.00  3.3852E-003
 383.00  4.2065E-003
 384.00  6.1745E-003
 385.00  8.8286E-003
 386.00  4.2646E-003
 387.00  0.0000E+000
 388.00  5.2739E-003
 389.00  8.0110E-003
 390.00  4.6509E-003
 391.00  5.4275E-004
 392.00  3.7508E-003
 393.00  1.0168E-002
 394.00  1.0653E-002
 395.00  9.2924E-003
 396.00  7.9877E-003
 397.00  1.0248E-002
 398.00  1.2157E-002
 399.00  1.1787E-002
 400.00  1.2336E-002





This graph was made from the reading above plus the wavelengths from 400 to 1099nm.  


Zoom






Photo of the tent with the white 4000K fixture and the spectrometer in my home lab.