Why UCU’s call on minimum ventilation standards is correct

Andrew Chitty, University of Sussex

Please see end of this post for additional notes added since publication.

In the context of emerging information about the new variant of Covid-19, UCU has called for employers to implement a minimum ventilation level of 12–17 litres per second per person of fresh air for typical offices and educational settings (UCU, 8 Jan 2021). This post explains how UCU’s call follows directly from recent recommendations made by SAGE’s Environmental and Modelling Group and as such is justified.

1. Background: airborne transmission of Covid-19 and the new variant

Airborne (or aerosol) transmission of Covid-19 is now widely recognised as a third route alongside large droplet transmission and transmission via contaminated surfaces. See for example the US CDC’s scientific brief SARS-CoV-2 and Potential Airborne Transmission (CDC, 5 Oct 2020) and the FAQs on Protecting Yourself from COVID-19 Aerosol Transmission (Marr et al, 9 Dec 2020). Whereas two metre social distancing and masks are the most effective measures against large droplet transmission, ventilation is the key additional measure needed against airborne transmission. Ventilation becomes especially important in the context of the spread of the new variant of the SARS-CoV-2 virus (called VOC 202012/01). PHE currently estimates this variant to be 30% to 50% more transmissible than previous forms, and it comprised some 90% of new cases sequenced in London, the South East and the East of England at the end of December, with the proportion of new variant cases rising rapidly towards the same level in the other English regions (PHE, 8 Jan 2020, p. 14 and Figure 5).

Accordingly the new government guidance Higher Education: Reopening Buildings and Campuses (DfE, 7 Jan 2020) says:

You should ensure that all indoor and covered areas have good ventilation in addition to other methods of risk reduction. This can help reduce the risk of spreading coronavirus (Covid-19) by aerosol transmission, so focus should be given to improving general ventilation. Poorly ventilated buildings are particularly conducive to virus spread.

However neither the guidance nor the Health and Safety Executive page to which it links, Ventilation and Air Conditioning During the Coronavirus (Covid-19) Pandemic (HSE, last reviewed 3 Dec 2020), specifies a minimum ventilation level.

In a press release of 5 January, Essential Workers Face Major Covid-19 Risk because of Outdated Safety Rules, the TUC called on the government to update workplace safety rules in light of the current understanding of the importance of airborne transmission, and in light of the emergence of new virus strains. Among other things, the TUC called on the government to “set a safety threshold for ventilation of indoor workplaces with outside air — CIBSE recommends at least 10 litres of outside air in offices per second per person” (TUC, 5 Jan 2021).

CIBSE (the Chartered Institution of Building Services Engineers) sets a ventilation standard of 10 l/s/person for new-build “executive, general and open plan offices” in its Guide A — Environmental Design (CIBSE, 2015, summarised at Clark, 29 Jul 2013, Table 3). This is also the standard that the Building Regulations 2010 part F sets for offices (HM Gov, 2013, Table 6.1b).

In the most recent version of its CIBSE Covid-19 Ventilation Guidance (version 4, of October 2020), CIBSE says that the level of ventilation “deemed adequate” for “typical offices” is 10 l/s/person of outside air, implying that during the pandemic this new-build standard should be applied generally (CIBSE, 23 Oct 2020). However CIBSE has not updated its advice since then. [1]

2. UCU’s call on minimum ventilation standards

In its Updated Covid-19 Guidance for Branches, released on 8 January, UCU went further than the TUC. In light of the emergence of the new variant of Covid-19 and the rising level of infections, it advised branches to ensure that employers, among other measures, “set a safety threshold for ventilation of 12–17 litres per second per person for typical offices and educational settings” (UCU, 8 Jan 2021).

My understanding, informed by conversations with UCU’s national office, is that this figure of 12–17 l/s/person is based on the following statements by SAGE’s Environmental and Modelling Group (EMG).

(1) In its 22 July 2020 paper Role of Aerosol Transmission in Covid-19, EMG affirmed the CIBSE ventilation threshold of 10 l/s/person:

The building regulations and guidance provided by CIBSE recommends 10 l/s/person fresh air is provided in most public and commercial buildings […] It is recommended that ventilation in all occupied public spaces should at least meet the current building regulations for the space. Those spaces which are occupied by more than one person and do not meet this standard should be have their ventilation upgraded where possible, or the use of the space restricted to a lower occupancy to reduce overall risk of transmission. (EMG/NERVTAG, 22 Jul 2020; see also EMG, 16 Dec 2020)

(2) In its 30 September 2020 paper Role of Ventilation in Controlling SARS-CoV-2 Transmission, EMG advised a slightly lower threshold of 8–10 l/s/person for “most workplaces and public environments”:

In most settings the risk of aerosol transmission is likely to be low if the ventilation rate achieves current design standards (medium confidence). For most workplaces and public environments this equates to a flow rate of 8–10 l/s/person based on design occupancy, although guidance for some environments allows for lower flow rates of 5 l/s/person. (EMG, 30 Sep 2020)

(3) However in its 23 December 2020 paper Mitigations to Reduce Transmission of the New Variant SARS-CoV-2 Virus, EMG recommended that in light of the spread of the new variant its earlier thresholds should be raised by a factor of “potentially” 1.5-1.7:

As a precautionary measure it is recommended that ventilation rates stated in previous EMG papers are adjusted accordingly to account for the increased risk (potentially 1.5–1.7 times higher). Application of ventilation controls is currently very variable between different settings, and it is essential to ensure that all public and workplace spaces include ventilation as part of their Covid secure risk assessment, and adopt appropriate measures to ensure it is effective (high confidence). (EMG/NERVTAG/Transmission Group, 23 Dec 2020)

Combining the figure of 8–10 l/s/person from the 30 September paper with the recommendation to increase ventilation by a factor of 1.5–1.7 in the 23 December paper leads to the conclusion that the safety threshold EMG now recommends is between 1.5 x 8 and 1.7 x 10, i.e. between 12 and 17 l/s/person, for most workplaces and public environments.

3. Conclusions

UCU’s call, in response to the emergence of the new variant of Covid-19, for a ventilation threshold of 12–17 l/s/person (of fresh air) is supported by EMG’s most recent recommendations. It follows that universities and FE colleges must not only, as EMG says, “ensure that all public and workplace spaces include ventilation as part of their Covid secure risk assessment”. They must further as a matter of urgency incorporate the 12–17 l/s/person threshold into their risk assessments for these spaces. Until this has been done universities and colleges should keep the spaces closed. Their moral and legal duties to staff, students, the wider community, and an NHS which is under serious strain require no less.

[1] Some authorities specify ventilation standards in terms of Air Changes per Hour (ACH). See for example the figures in the US CDC’s Guidelines for Environmental Infection Control in Health-Care Facilities (CDC, Jul 2019, p. 50). However litres per second per person is a better metric, since the adequate level of ventilation for a space depends on how crowded the space is. This is the metric used by both CIBSE and the Building Regulations.

Additional notes

(1) The standard formula to convert from air changes per hour to litres per person per second is:

Ventilation rate (litres/second/person) = air changes per hour x volume of the space (cubic metres) x 1000 / 3600 / number of occupants

Here the 1000 converts cubic metres to litres and the 3600 converts hours to seconds.

(2) The September EMG paper ‘Role of ventilation in controlling SARS-CoV-2 transmission’ (EMG, 30 Sep 2020) has a helpful section on the use of CO2 sensors to measure ventilation rates. See Q4. The maximum level of 800 ppm of CO2 recommended there corresponds to 15 litres per second per person according to Figure 2 in the CIBSE Journal module ‘Delivering ventilation to occupied spaces’ (CIBSE, October 2014).

(3) I am keeping a crowdsourced document of links on airborne transmission of Covid-19 and prevention measures against it (including ventilation) at https://docs.google.com/document/d/1UbuInsDTSqPmYt63WsCniWQxDxsQywUnD0k7jaJU7PI/edit.

4 March 2021

The author is Health and Safety Officer for Sussex UCU. This blog is written in a personal capacity. Many thanks to Alex Lancaster and Jackie Grant. All errors are the author’s own. Comments welcome by Twitter to @aechitty1.

Senior Lecturer in Philosophy, University of Sussex

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