BioVigilant Systems Frequently Asked Questions

Military and Homeland Security

Q: Can your detector truly detect particles smaller than the wavelength of the laser light you are using?

A: Yes. Our particle detector is based on the Mie scattering principle. Mie scattering occurs when the sizes of the particles being scattered are comparable to the wavelength of the light. Our detector detects the scattered light intensity from the small particles, which in the Mie scattering regime is sensitive down to 0.5 micron.[1] It shall be noted that we are not trying to image the bio-particles by microscopy technique, in which case the detection limit of the particle size is set by the wavelength of light in the so-called diffraction limit. This diffraction limit is quantified by the Rayleigh’s Criterion as follows: d= 1.22lf/D=0.61l/N.A. where d is the smallest distance between two objects resolvable by a microscope, l is the wavelength of light, f is the focal length of the microscope objective lens, and D is the diameter of the aperture of the microscope and N.A. is the numerical aperture of the microscope.[2] In our Mie scattering technique, we simply detect the scattered light intensity to gain information of the size of the airborne particles.

Q: What is the particle size detection range of your particle detector? And what is the sensitivity of the detector?

A: Our detector can detect and classify airborne particles in the size range of 0.5 micron to 50 microns. Our detector is a single event particle counter, it can detect the presence of a single airborne particle passing through the detector sensing region. Its detection sensitivity is 1 aerosol per liter of air. Please note that this detection sensitivity is determined by the sampling fan used in our current design and can be enhanced by using a more powerful fan.

Q: Your detector is based on laser diode. The output wavelength of a laser diode can change with ambient temperature. Is your detector affected by a wavelength change?

A: No. In the Mie scattering regime, the scattering angular distribution and scattering intensity are both weak functions of wavelength. In fact, for sufficiently large particles there is no wavelength dependency at all.[3]

Q: Your detection scheme is based on the monitoring of particles within the weaponized bio-particle size range from 1 to 5 microns. Will naturally occurring dust, smog and pollen particles in the air confuse your detector?

A: No. There are very few naturally occurring airborne particles in the size range of interest, i.e. 1 to 5 microns. Smog particles are in the size range of 0.4 to 0.6 micron.. Pollen and other allergen sources are in the 5 to 50 micron range. Precisely because of the scarcity of naturally occurring particles in the 1 to 5 micron range, the human body has not evolved to build a defense against particles in this size range. This is the reason why weaponized bio-particles are very dangerous: they can go right into the lungs.

Q: Would dust storms or pollen season confuse your bio-particle detector?

A: No. First of all, weaponized bio-particles (such as anthrax spores) are in a different size range from other airborne particulates. Second of all, it will be inefficient for the terrorists to release bio-agents during a dust storm or in a pollen season. Large airborne particles (e.g. pollens) tend to scavenge weaponized bio-agents, i.e. getting the bio-agents attached to larger particles, thus rendering them ineffective in gaining passage into human lungs.[4] Therefore, the confusion caused by atmospheric conditions is not likely to occur.

Q: Is your detector sensitive enough to give adequate early warning of a bio-terrorist attack?

A: Yes. Our detector is sensitive in detecting the presence of airborne bio-agents. Under normal conditions, there are few particles in the size range of interest (1 to 5 microns.) When a bio-terrorist agent is released, a sudden jump of the particle concentration will occur in that range, say, from 10 particles per liter of air to several hundreds of particles per liter of air. Our detector is fully capable of giving warnings to such a jump in particle concentration. According to a simulated study by Defence Research Establishment Suffield of Canada, when letter-borne anthrax spores are released, there will be a large jump in the airborne particle concentration, readily detectable by a particle counter.[5]

Q: What is the lethal dosage of inhalation anthrax spores?

A: The currently established lethal dosage for adult human LD50 (lethal dosage to kill 50% of exposed persons) is 8,000 to 50,000 spores.[6]

Q: Why is an early warning device useful as a countermeasure for a bio-terrorist attack?

A: “Biological Detection and Surveillance Buys Time” is the banner slogan of Department of Defense’s anti-terrorist effort.[7] There is a 1 to 3 day incubation period for anthrax spores. If the bio-terrorist attack is detected early, antibiotics can be administered to the exposed persons to eliminate the inhaled spores. The treatment is exceptionally effective if early detection is made during the spore incubation time.

Q: Will your detector be a suitable early warning device?

A: Yes. Our detector can detect an abnormal spike in airborne particle concentration. It will give out a warning signal when suspicious concentration spikes are detected. It will prompt a more elaborate biological testing to identify the type of the agent. Our detector is a cost-effective way to implement an early warning bio-attack system. The maintenance-free continuous operation of this type of early warning detector will enhance the level of security and, at the same time, reduce the cost of manpower and resources in routine biological testing.

Q: Why are weaponized bio-agents are particularly dangerous to humans?

A: Weaponized bio-agents are harmful bacterial spores (such as anthrax) processed into a size range that enables them to gain passage into human lungs and get lodged there. When the spores germinate after an incubation time of 2-5 days, the vegetative bacteria release toxins to cause inflammation of the lung tissue and subsequent death by suffocation. The key to an efficient bio-weapon is the access to the inner parts of human lung. The following diagram from a EPA circular illustrates the relationship between particulate size and the level of penetration in the human lung.[8]

Q: What are the airborne particles and their size ranges?

A: The following graph gives a rough description of airborne particles and their size ranges:.

Q: Your airborne particle detector has a range covering 0.5 to 50 microns. Can it be used in applications other than bio-agent detection?

A: Yes. Our detector can be used for industrial air quality monitoring, general environmental monitoring (e.g. pollen count reporting and airborne particulate recording) and consumer allergen detectors.

[1] H.C. van de Hulst, “Light Scattering by Small Particles” page 406, Fig. 94, Dover , 1981.

[2] Warren J. Smith, “Modern Optical Engineering”, pages 135-141, McGraw-Hill 1966.

[3] J.R. Meyer-Arendt, “Introduction to Classical and Modern Optics”, p. 301, Prentice Hall 1989.

[4] Philip J. Wyatt, “Early Warning and Remediation: Minimizing the Threat of Bioterrorism”, Journal of Homeland Security, April, 2002.

[5] B. Kournikakis, S.J. Armour, C.A. Boulet, M. Spence, and B. Parsons, “Risk Assessment of Anthrax Threat Letters” Defence Research Establishment Suffield, Technical Report: DRES TR-2001-048, September 2001, http://hs.cupw.ca/pdfs/anthrax_threat_letter_eng.pdf

[6] Jim Davis, and Anna Johnson-Winegar, “The Anthrax Terror: DoD’s Number-One Biological Threat”, Aerospace Power Jopurnal, Winter 2000.

[7] Anna Johnson-Winegar, Asst. Sec. Of Defense for Chemical and Biological Warfare, Missouri Conference on Homeland Security, Aug. 8, 2002.

[8] EPA/625/R-96/010a, “Continuous Measurement of PM10 Suspended Particulate matter (SPM) in Ambient Air”, June, 1999, http://www.epa.gov/ttn/amtic/files/ambient/inorganic/overvw1.pdf

Contact BioVigilant Systems at info@biovigilant.com or call (520) 292-2342.