Sunday, November 01, 2020


CAMS[1] is the project that aims to sequester expired air from Covid-19 patients from the room where they are being nursed in order to prevent infection of other patients and front-line health care workers (HCWs).



  1.            Covid-19 is spread by droplets, aerosols and touch
  2.           Patients suffering from Covid-19 (C19) emit infectious droplets and aerosols
  3.           Viruses in these aerosols can remain viable for hours
  4.           Droplets and aerosols contribute to Hospital Acquired C19 Infections (HACIs)
  5.           HACIs form a significant part of NHS hospital work
  6.           If expired air from Covid-19 patients is collected and filtered, the risk of HACIs must diminish
  7.            High Efficiency Particle Absorbing (HEPA) filters can capture C19

Demonstrating the effectiveness of CAMS in practice:

8               When CAMS is rolled out, longitudinal audits should provide evidence of lessened HACIs

            Audits may also provide evidence of benefit to C19 patients


The Evidence

“Droplet” refers to watery particles containing virus that fall to the ground within 2 metres; they are defined here as of size greater than 100 microns. Aerosol refers to particles of size less than 100 microns; they can remain suspended in the air for hours and can concentrate in still air.

It used to be thought that Sars-CoV-2 (the virus that causes Covid-19 disease) spread only through droplets and touch transfer. Recent work shows that aerosols are more important, and the WHO now accepts that aerosols are an important means of transmission[2].

Jianxin Ma[3] et al found that patients in earlier stages of C19 exhaled millions of Sars-Cov-2 units per hour. Ma found that “air sample data, despite the low positive rate, still show that the air in the hospitals that housed the COVID-19 patients was contaminated with SARS-CoV-2”.

Van Doremalen found that aerosolised Sars-CoV-2 may remain viable in the air for up to 3 hours[4].

Lednicky et al. found that viable (infectious) SARS-CoV-2 was present in aerosols within the hospital room of COVID-19 patients, that airborne virus was detected in the absence of healthcare aerosol-generating procedures, and that the virus strain detected in the aerosols matched with the virus strain isolated from a patient with acute COVID-19[5].

Jianxin Ma[6] states that “None of the 26 surface swabs collected from handles of various objects appeared positive for the virus (Supplementary Table 4). These observations seem to not support the widely held belief that direct transmission by contact with surfaces plays a major role in COVID-19 spread”[7].

If touch is less important in our estimation as a means of transmission, it follows that droplet and aerosol spread must become more important, given that there is no other known means of transmission.


Hospital Acquired Covid-19 Infections

A significant number of cases of Covid-19 are contracted in hospital. They form about 10-20% of hospitalised cases.

There are a large number of reports detailing nosocomial (hospital acquired) Covid infections, and the following is just a selection of some of the cases of known HACIs:

1.       Weston General Hospital, May 2020: 57 Hospital Acquired infections (HACIs). 18 patients died[8].

2.       University College Hospital 2nd March-12 April, 2020: of 435 C19 inpatients, 15% were HACIs. 24 died[9].

3.       Dublin: 50 cases, 14 died. March – May 2020[10]

4.       Manchester Tameside General Hospital, 18 cases of HACI September 2020[11]

5.       West Glamorgan Hospital: 60 C19 HACI’s , 8 die[12]

6.       12.5% of hospital C19 patients contracted the infection in hospital (n=1,564)[13].

7.       Up to 20% of hospitalised C19 patients caught it in hospital[14].

Here is a list of links to other papers instancing HACIs:  


It is clear that nosocomial infection is a serious problem with Covid. It causes problems for both patients and staff. Infection in staff reduces the supply of skilled medical and nursing personnel, which puts colleagues under stress. Stressed people are more vulnerable to infection, which may explain why so many medical and nursing staff of working age, who might be expected to have a mild form of Covid-19, have in fact died.  36 NHS doctors have died, but the number of NHS nurses who have died is difficult to establish. The International Council of nurses says that 1,500 nurses have died worldwide[15]. It is reported in August that the Covid-related deaths of more than 620 front-line NHS and social care staff are to be investigated[16].

It is important that we consider seriously any technology that may lead to reducing HACIs.

Masks are helpful both for preventing spread from and to the mask wearer, but by no means a perfect defence against infection.

Southampton University has developed a system, PerSo, that filters air and supplies it to the HCW in a hood. It was developed in one week, and has been under official assessment for some five months[17].


Technologies designed to reduce HACIs

Professor Monty of the department of the Department of Mechanical Engineering at Melbourne University has developed a personal ventilation hood[18] that filters all exhaled air from a Covid-19 patient. It is on the verge of being available on the market.

A similar technology is being developed in Minnesota, the Aerosol Hood, with a rigid hood fitted with iris ports to allow HCWs to access the patient[19].


Effectiveness of Filters to capture Viruses

Here[20] is a useful background to HEPA filters which are in routine use in civil airliners.

Zhang et al[21] found that high-efficiency residential HVAC (Heating, Ventilation and Air Conditioning) HEPA filters were effective at capturing airborne virus particles in the air passing through the filter.  The filters tested were HEPA standard, and the virus used in their experiments were significantly smaller than Sars-CoV-2.

NASA work on spacecraft air purification confirms the efficacy of HEPA and carbon filters in removing particles smaller than Sars-CoV-2[22].

The above should establish the desirability of providing hospitals and care homes with one CAMS unit for every C19 patient.

Audit evidence

When CAMS or similar technologies are rolled out, it is to be expected that the levels of HACIs in the institution will be reduced. This hypothesis can be tested by comparing the rate of infection in front-line HCWs before the CAMS is introduced and after it is introduced. The experiment could be controlled by removing the filter from the system, but that would clearly be unethical.

A second hypothesis is that a patient suffering from C19 who is continually reinfecting themselves with the virus by rebreathing aerosols and touching fomites (infected surfaces such as their own bedclothes) will take longer to recover than a patient who is not rebreathing virus to the same extent. Examination of outcomes from patients who are given CAMS technology will test this hypothesis.


Cost of one CAMS unit

A rough estimate for one unit:


                Hood                     20

                Tube                      5

                HEPA filter          30

                Fan                         10

                Labour                  50

    Marketing           50  

                TOTAL                   165




There can be no reasonable doubt that sequestering droplets and aerosols emitted from C19 patients will reduce the probability that health care workers attending these patients, and people breathing air in the same building as those patients may contract C19.

R H Lawson





[4] van Doremalen N, Bushmaker T, Morris DH, et al.   Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med 2020; 382 : 15647




[21] Study of Viral Filtration Performance of Residential HVAC Filters.  Zhang, John; Huntley, Doug; Fox, Andy; Gerhardt, Bryan; Vatine, Al; Cherne, John. ASHRAE Journal . Aug2020


[22] Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials J.L. Perry et al.




© R Lawson

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