This is crucial. Bring more fresh filtered outdoor air into the building. If mechanical ventilation options are not available, open your windows and doors as much as possible. This can help extract and reduce airborne contaminates, making infection less likely. This needs to be done in a controlled manner, not through random holes in the building envelope, which can increase exposure to toxic VOCs, construction particulates, pests, and/or chemicals from garages and crawlspaces. Since windows can only be opened so much throughout the year, it becomes critical to ensure a controlled mechanical ventilation strategy is included in the building design. This is a topic that has only recently been addressed by the code councils and has had much more consideration in larger commercial buildings.

The concept of outside air or ventilation air is still somewhat of a novelty to many builders, homeowners, and HVAC contractors in their understanding of how it relates to a healthy home. Building science principles and the codes that come from them are usually an afterthought for most builders and homeowners with houses built before there were such standards. 

Indoor Air Quality (IAQ) experts are now more than ever recommending increasing the fresh air being brought into the homes as recommended by the maximum that code or ASHRAE's guidelines propose if possible. If known biologic contamination is present increasing this, even more, should be considered. Hospitals ventilate at a rate of up to 12 air changes an hour.

In a study from 2019, it was found that ensuring even minimum recommended levels of ventilation can be as effective for preventing influenza as having 50% to 60% of the folks in the building vaccinated! 

We are starting to see an interest in supply-side ventilation systems that do not rely on the air handler itself for ventilation air but does the ducting to distribute this air to all rooms of the home. The advantage of this is bringing in fresh air from a known, outside source and not from random holes in the building as most exhaust ventilation strategies do. This air can then be filtered as it is brought into the home and then tempered by the duct system as it is distributed throughout the home. These can also be fairly energy efficient using 30-50 watts, much less than the 200-1000 watts using the air handler.

Another interesting consideration is to increase and/or keep relative humidity (RH) between 35% - 60%. This is important because 80% of droplet nuclei of bacteria and viruses deteriorate in this humidity level. Lower and higher levels of RH can increase the viability of biological contaminates. An important caveat to remember is that condensation can occur on the inside of windows and cold surfaces depending on inside and outside temperatures and should be monitored.

Careful monitoring of the RH is recommended at all times of the year. Please see the chart below to review how RH relates to various indoor air contamination.

Air purification is also worth considering in high contact areas in buildings and homes. Studies have shown a dramatic increase in asthma and other respiratory conditions in homes with high particulate counts. The particulates also give away for biological contaminates to attach and be distributed throughout the home. Many portable EPA approved HEPA filtration systems can filter 99.9% of particulates.

Wirecutter published a great review of many consumer-available models to consider. Some of the available filters can also be effective during our increasingly frequent wildfire seasons. 

Note: If higher Minimum Efficiency Reporting Value (MERV) filtration is added to the HVAC air handler system, care should be taken to see if the system is designed for these filters. An HVAC professional should be consulted as these can greatly reduce airflow in systems causing potential heating and cooling performance issues.

This clever design below can quickly and cost-effectively be used in many indoor areas.