Respiratory Viruses Sensitive To Mild Acidification Of Ambient Air

The US is presently in the midst of a triple-demic of Covid-19, Influenza, and RSV cases, all of which spread via airborne transmission while preventative interventions like mask-wearing are at an all-time low.

Infections spread by aerosol transmission are incredibly challenging to control. Aerosols carrying infectious viruses can linger for hours in the air after they have been exhaled. While vaccines are highly effective in reducing the risk of death and serious illness, they are generally not as effective in preventing transmission. In our current climate, the best way to reduce the risk of transmission is to reduce the concentration of airborne virus that is available to be inhaled and can therefore cause infection.

Air filtration and ventilation, and Far UVC germicidal light are existing tools that can mechanically reduce the concentration of airborne viruses and don’t require behavior change. Now, a new Swiss study has discovered that certain levels of acidity can inactivate different viruses, providing a new tool to control airborne transmission.

Many viruses, such as the influenza A virus, are acid-sensitive. Exhaled aerosol particles can absorb volatile acids and other airborne substances, such as acetic acid, nitric acid, or ammonia, from the indoor air. This affects the acidity levels of the particles and the viral load they carry. The chemical environment surrounding aerosolized viruses, which determines the aerosol pH, is an understudied area of virus mitigation.

The study tested both the Influenza A virus and the SARS-CoV-2 Beta variant in synthetic lung fluid and mucus harvested from primary epithelial nasal cultures grown at the air–liquid interface and found that the exhaled aerosols acidified more quickly than expected, yet the speed was dependent on the concentration of acid molecules in the ambient air and the size of the aerosol particles. Virus inactivation curves were measured at room temperature in 2 mL glass vials, 500 μL PCR tubes, or 1.5 mL plastic tubes using a matrix volume between 10 μL and 1 mL. Each experimental condition was tested in triplicate, except for a subset of SARS-CoV-2 experiments, which were performed in duplicate.

Both viruses had very different acid sensitivities in terms of inactivation. It took a pH of below 2 to inactivate the SARS-CoV-2 Beta virus. Such conditions are rarely reached in typical indoor air as aerosol pH hardly ever falls below 3.5. The Influenza A virus, however, was inactivated after just one minute in acidic conditions of pH 4. The researchers concluded that the rapid acidification of the aerosols was largely due to nitric acid that has entered from the outside air. Nitric acid is typically formed by the chemical transformation of nitrogen oxides. Which are released into the environment as a result of combustion processes along with the exhaust gases of diesel engines and domestic furnaces. It often enters through open windows or when ventilation systems draw outside air in.

However, inactivation times for both the Influenza A virus and SARS-CoV-2 can be greatly reduced if the indoor air is slightly acidified. This can be achieved by either removing basic gases or adding acidic ones. The gaseous acid molecules also need low volatility to transition from the gas phase to the condensed phase readily, and once dissolved, they must be sufficiently strong acids to overcome any pH buffering by the particle matrix. The researchers found that if the indoor air is enriched with nonhazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99%-inactivation times for both SARS-CoV-2 and Influezena A in small aerosol particles to below 30 s.

The results of this study demonstrate that an effective reduction in viral load and consequently transmission can be achieved via air acidification, more specifically by applying nitric acid at levels lower than 10% of the legal exposure thresholds. As acid exposure is below the legal limit, it will likely not cause harmful effects on human health. However, further research should investigate the consequences of acid accumulation in indoor air on the microbiome and immune response in the respiratory tract. Real-time monitoring of aerosol pH in indoor spaces to prevent acid overexposure and to ensure efficient virus inactivation would also be necessary.

Despite the current unknowns, this new discovery could have a profound impact on infectious disease mitigation strategies.