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Scientific foundations

Decades of research have shown the importance to health and safety of ventilation in public spaces. We recognise the importance of all available barriers in the defence against airborne pathogens such as the SARS-COV-2 virus but our focus is on ventilation  –  the key, but often ignored defense against disease transmission. Proper ventilation ensures the circulation of clean air through an indoor space, thereby clearing out pollutants. Assessment includes measuring carbon dioxide concentrations in specific areas of occupied indoor spaces as it has been shown that the carbon dioxide levels are a good proxy for the levels of other possible contaminants in the air. GreenFlag certification is issued if ventilation controls are adequate.

Why use ventilation as the cornerstone criteria for GreenFlag certification?

  • Ventilation is the most neglected barrier to the spread of disease
  • 25% of COVID-19 infections are linked to the fomite route
  • Studies show lower transmission infections attributed to the fomite route (about 16%)
  • Aerosol transmission of COVID-19 is probably the most important route of transmission
  • Virus-laden aerosols frequently infect susceptible contacts at close proximity where they are most concentrated — like smoke
  • The main “universal” barriers to transmission in public spaces now importantly include ventilation

CO2 is a surrogate for estimating Covid-19 infectious dose in an occupied space

High indoor space CO2 levels correlate with other measures of poor indoor air quality, and are linked to increased absenteeism, loss of productivity and poor decision-making.

Green Flag CO2
  • CO2 measured in parts per million (PPM) is a measure of ventilation status of an indoor space

  • 800 parts per million CO2 roughly equates to a fresh air ventilation rate of 15 liters per second per person

  • A CO2 value maintained less than 400ppm above ambient (outside) air limits viral transmission (99.9% risk reduction)

  • Outdoors, CO2 measures about 420 PPM. Any additional CO2 in an indoor space is usually generated by occupants of that space

Ventilation is the “universal” barrier to transmission in public spaces is ventilation.

Air change rates required to maintain a given CO2 concentration in a typical office with 20 people breathing normally.
Air change rates required to maintain a given CO2 concentration in a typical office with 20 people breathing normally.

What is airborne particulate matter?

Airborne Particulate Matter (PM) is a complex mixture of solid and aerosol particles
composed of small droplets of liquid, dry solid fragments and solid cores with liquid coatings that vary in size. Particles are defined by their size for air quality regulatory purposes. Those with a diameter of 10 microns or less (PM10) are inhalable into the lungs and can induce adverse health effects. Fine particulate matter is defined as particles that are 2.5 microns or less in diameter (PM2.5).
PM10 and PM2.5 often derive from different emissions sources. Emissions from combustion of gasoline, oil, diesel fuel or wood produce much of the PM2.5 pollution found in outdoor air. PM10 includes dust from construction sites, landfills and agriculture, wildfires and brush/waste burning, industrial sources, wind-blown dust from open lands, pollen and fragments of bacteria.

Both PM2.5 and PM10 can be inhaled, with some depositing throughout the airways.

Particles deposited on the lung surface can induce tissue damage, and lung inflammation. PM2.5 is associated with the greatest proportion of adverse health effects related to air pollution.

Short-term exposures to PM10 have been associated primarily with worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalisation and emergency department visits.

Long-term (months to years) exposure to PM2.5 has been linked to premature death, particularly in people who have chronic heart or lung diseases, and reduced lung function growth in children.

Useful resources

Read more about PM2.5 and PM10 here:,5)

Read more about the adverse health effects of PM2.5 and PM 10 here:

AIr Pollution
PM2.5 and PM10 are microscopic airborne particles that can be hazardous to people's health.