We have found that a lot of workplaces and homes have poor ventilation. Take the modern home, unless you make a conscious decision to put in a ventilation system there will be none.

My younger brother said he always keeps his sons door closed at night. So I set up a CO2 monitor in his room overnight. The concentrations by morning were well over 2000 ppm. He kept the door and a window open from that day on. New houses are often well sealed so you will not get any ventilation.

The bugs and fumes from building materials will off gas and circulate.

Then offices - a lot of building owners and tenants will have a high proportion of recycled air. That is often to retain the heat. The typical situation is the tenant has no clues on how much fresh air is coming into the offices. It is often maintained and run by an air conditioning company. Often it is not seen as an issue.

The filters in the air conditioning system often are not serviced and maintained to a standard required. They will not remove the fine particulate that has the greatest health effect.

So what to do, if you are going to rent an office
-stipulate in the lease the minimum air quality requirements.
- if you have a property you occupy then know how much air changes per hour are happing.
- know what the ratings of the filters in the air conditioning system and check to see they are working.
- do not accept that these issues will be managed well by the owner, sometimes they are not.

For houses
- Open the doors and windows
- If you have extraction systems - use them
- Get an air exchange system - there are lots about
- The best air exchange system recovers heat from the stale air.
- Open doors allow air flow.

If you are looking at air treatment as an engineering control to suppress Covid 19 and/or other airborne pathogens, the Global Lighting Association has released the following guideline.

Guidelines for quantification of airborne pathogen inactivation by UVGI technologies
This document outlines a methodology for determining the microbial cleaning capabilities of UVGI products. It is particularly relevant in the battle against the SARS-CoV-2 virus.

Last year the European Centre for Disease Prevention and Control (ECDC) prepared guidance for public health authorities in EU/EEA countries and the UK on the ventilation of indoor spaces in the context of COVID-19xxi. This guidance is targeted at public health professionals and serves as a basis for REHVA to provide technical and system-specific guidance for HVAC professionals. The main evidence and conclusions by ECDC can be summarised as follows:
• The transmission of COVID-19 commonly occurs in enclosed indoor spaces.
• There is currently no evidence of human infection with SARS-CoV-2 caused by infectious aerosols distributed through the ventilation system air ducts. The risk is rated as very low.
• Well-maintained HVAC systems, including air-conditioning units, securely filter large droplets containing SARS-CoV-2. COVID-19 aerosols (small droplets and droplet nuclei) can spread through HVAC systems within a building or vehicle and stand-alone air-conditioning units if the air is recirculated.
• The airflow generated by air-conditioning units may facilitate the spread of droplets excreted by infected people longer distances within indoor spaces.
• HVAC systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing the recirculation of air, and increasing the use of outdoor air.
• Building administrators should maintain heating, ventilation, and air-conditioning systems according to the manufacturer's current instructions, particularly concerning the cleaning and changing of filters. There is no benefit or need for additional maintenance cycles in connection with COVID-19.
• Energy-saving settings, such as demand-controlled ventilation controlled by a timer or CO2 detectors, should be avoided.
• Consideration should be given to extending the operating times of HVAC systems before and after the regular period.
• Direct air flow should be diverted away from groups of individuals to avoid pathogen dispersion from infected subjects and transmission.
• Organizers and administrators responsible for gatherings and critical infrastructure settings should explore options with the assistance of their technical/maintenance teams to avoid the use of air recirculation as much as possible. They should consider reviewing their procedures for the use of recirculation in HVAC systems based on information provided by the manufacturer or, if unavailable, seeking advice from the manufacturer.
• The minimum number of air exchanges per hour, following the applicable building regulations, should be ensured at all times. Increasing the number of air exchanges per hour will reduce the risk of transmission in closed spaces. This may be achieved by natural or mechanical ventilation, depending on the setting.

In the guidelinexxxiv ECDC stresses the importance of ventilation by concluding that ensuring the implementation of optimal ventilation adapted to each particular indoor setting could be critical in preventing outbreaks and transmission amplification events. In the guideline the minimum number of air exchanges per hour, in accordance with the applicable building regulations, is required to be ensured at all times. It is stated that increasing the number of air exchanges per hour, by means of natural or mechanical ventilation, will reduce the risk of transmission in closed spaces. Ventilation has seen as a major method because there is no evidence on the effectiveness of methods for decontamination of air (e.g. UV light irradiation) for use in community settings.

Thank you for that Julie,
Interesting to note the UV light irradiation on those air purifiers might not be so effective, compared to what they are claiming on the sales pitch.
Probably same goes for the HEPA filters on these portable air purifiers. Most of them are tiny filters used in a large open plan office, compared to large filters in a ducted HVAC system.

Also read user feedback of some of those cheaper CO2 monitors out there are giving random varied readings (when they are placed side by side). Worried how accurate these things are, as they are meant to be a tool to indicate how stuffy the room is.

• There is currently no evidence of human infection with SARS-CoV-2 caused by infectious aerosols distributed through the ventilation system air ducts. The risk is rated as very low.
• Well-maintained HVAC systems, including air-conditioning units, securely filter large droplets containing SARS-CoV-2. COVID-19 aerosols (small droplets and droplet nuclei) can spread through HVAC systems within a building or vehicle and stand-alone air-conditioning units if the air is recirculated.
• The airflow generated by air-conditioning units may facilitate the spread of droplets excreted by infected people longer distances within indoor spaces.
• HVAC systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing the recirculation of air, and increasing the use of outdoor air. — Julie

I'm struggling to reconcile some of these claims: No evidence of infection via aerosols distributed through ventilation. Aerosols can spread through HVAC systems if air is recirculated.

What have I misunderstood?

Is the important difference between ventilation (used air is deposited outside and new air comes into the system) versus re circulation (used air is reused)?

Is the important difference between ventilation (used air is deposited outside and new air comes into the system) versus re circulation (used air is reused)? — Peter Beaver

Yes, I think that is the crux of it Peter, although for a belt and braces approach, I would look at UV Technology also.