How to Improve Air Quality in Homes? (Technical Guide)


People are staying at home more frequently than ever before. As we spend more time indoors, air quality becomes a concern. I also work from home most of the time. So, I did some research on home air quality improvement.

It turns out that in order to improve home air quality, we need to keep the level of PM2.5 pollutants below 5 μg/m3 most of the time. Furthermore, the concentration of CO2 must be kept at below 1000 ppm to ensure that we have sufficient oxygen at all times.

Although maintaining a minimum fresh air supply rate at 15-20 cfm per person can improve home air quality, we must recognize that the outdoor air in certain regions of the world is unhealthy. Therefore, more efforts are needed to improve air quality in homes.

What is a Good Air Quality Level in Home?

To understand how to improve air quality in homes, we must first understand what is considered a good air quality level in homes.

In the year 2021, the World Health Organization (WHO) came out with a new guideline for air quality. Following is the summary of their recommendations for homes:

PollutantFormulaLong-TermShort-Term
Particulate Matter 2.5PM2.55 μg/m315 μg/m3
Particulate Matter 10PM1015 μg/m345 μg/m3
Nitrogen DioxideNO210 μg/m325 μg/m3
OzoneO360 μg/m3100 μg/m3
Carbon MonoxideCOHarmful4 μg/m3
WHO Recommended Air Quality Guideline (AQG) Level

From the above table, the long term means the average level in a year (6 months for O3) while the short-term means the average level in 24 hours (8 hours for O3).

Based on WHO recommendations, we basically need to keep the level of PM2.5 and PM10 below 5 μg/m3 and 15 μg/m3 respectively for most of the time. Meanwhile, we should also prevent them from rising above 15 μg/m3 and 45 μg/m3 respectively at any given moment.

Furthermore, the level of NO2 and O3 should be kept at below 10 μg/m3 and 60 μg/m3 respectively for most of the time while preventing it from rising above 25 μg/m3 and 100 μg/m3 respectively at any given moment.

Finally, the level of CO should be kept at below 4 μg/m3 all the time.

WHO has a full list of pollutants and the recommended level. The above table shows relevant ones for homes. If you’re interested to see the full list, download the 2021 air quality guideline (AQG) from the WHO website.

Now, let’s briefly go through each pollutant to better understand what are they:

  • Particulate Matter – Particulate Matter or PM is the most recognizable pollutant in home. Many air quality control organizations primarily use PM to determine if the air is healthy or unhealthy. The number beside a PM represents the size of the harmful particle. For instance, PM2.5 means the particle is 2.5 micrometers and smaller in diameter.
  • Nitrogen Dioxide – Nitrogen Dioxide or NO2 is produced primarily through combustion processes such as vehicle engine, cooking, furnace and gas boiler.
  • Carbon Monoxide – Carbon Monoxide or CO is one of the results of incomplete combustion processes. In homes, CO is mainly come from vehicle exhaust.
  • Ozone – Ozone or O3 is produced by certain appliances in home. Furthermore, introducing outdoor air into the house will bring in some amount of O3.

In addition, allergens, mold, fungus, bacteria and viruses are also present in the air in homes. Hence, we also need to get rid of them in order to improve the air quality in our home.

How to Improve Air Quality in Home?

As you have might guess already, in order to improve air quality in homes, we must keep all of the harmful pollutants below the recommended level as shown earlier.

There are two steps to control the level of pollutants in our home; 1) air purification and 2) air change. We need to adopt both for good home air quality.

Step 1: Air Purification

Air purification is a direct and effective method to control the level of pollutants in homes. Products such as air conditioners and air purifiers are practical solutions to improve air quality in homes.

However, basic air conditioning units such as wall-mounted units and ceiling cassette units are mostly equipped with low-grade air filters. So, we need to use ducted units or centralized units because they can be fitted with a higher grade filter.

On the other hand, air purifiers are meant to purify the air and improve air quality in homes. Most of the time, they are equipped with a HEPA or high-efficiency particulate air filter that can remove almost all PM2.5 and PM10 pollutants as well as allergens such as mold, fungus and bacteria.

Nonetheless, HEPA filters have limitations. They are not able to trap gases such as NO2, O3, CO and VOCs (volatile organic compounds). Hence, we need to use the air change method to dilute those harmful gases.

Step 2: Air Change

Air change is also known as air replacement. In short, we need to introduce outdoor air (fresh air) and exhaust out the “old air” in our house to dilute harmful gases.

ASHRAE recommended that homes should have a minimum ACH of 0.35 but not less than 15 cfm per person to ensure adequate indoor air quality.

ACH stands for air change per hour. ACH 0.35 means 35% of the air in a room is replaced by outdoor air per hour. Also, ACH 1 means that the entire air in a room is replaced by outdoor air every hour.

15 cfm per person means that the outdoor airflow rate must be at 15 cfm for every person in the room. For instance, if the living room has 6 persons, the outdoor airflow rate should be at least 90 cfm.

When outdoor air is introduced, it creates a positive pressure within the house. As a result, the “old air” within the house will get “pushed” out of the house through windows and doors gap.

Therefore, harmful gases such as NO2, O3, CO and VOCs within the “old air” will get exhausted out of the house and their concentration level will drop.

How to Purify Air in Home?

To purify air effectively, a HEPA filter is needed. HEPA filters are long proven to be effective in trapping PM2.5 and PM10 pollutants as well as allergens such as mold, fungus and bacteria.

Air Purifier

The best way to purify the air in homes is using an adequately-sized air purifier that has a HEPA filter. It’s best to have a low-grade primary filter before a HEPA filter to prolong the lifespan of the HEPA filter.

An example of a good air purifier is the Coway Airmega AP-1512HH True HEPA Air Purifier. Not only does it has a HEPA filter and a primary filter, but also has an additional layer of filter to remove VOCs.

When using an air purifier, it is extremely important that you have sufficient coverage. Each air purifier has a specific floor area coverage and it’s written on the product specification. Else, check with the seller.

Air Conditioner

If by any chance your air conditioner is compatible with a HEPA filter, by all means, install one to improve the air quality in your home.

Normally, home central air conditioners or air handling units are not compatible with a HEPA filter. However, some of them are compatible by default or through minor modifications. So, check with the manufacturer.

When you’re shopping for a HEPA filter, make sure that the MERV rating is above 17. Anything below MERV 17 is not a HEPA grade filter. To understand more, see my post 6 Types of Filters in HVAC (Efficiency & Applications).

How to Exchange Air in Home?

There are two ways to exchange the air in your home; a) natural ventilation and b) forced ventilation. Sometimes, natural ventilation is not suitable due to poor outdoor air quality.

Natural Ventilation

Natural ventilation basically means opening windows and doors to let outdoor air flows into your house and replace the “old air” naturally. However, it may not be suitable for certain regions of the world due to poor outdoor air quality.

You can visit IQAir.com to check your local air quality and see if natural ventilation is suitable for you. Many people thought that simply letting more outdoor air come into their house will improve air quality but that’s not always the case.

For example, India Delhi unfortunately is one of the places with very poor outdoor air quality. On 17 March 2022, the level of PM2.5 is 110 μg/m3 which is 22 times more than the WHO long-term recommendation.

As you can see, if people living in Delhi just follow the minimum air change rate without purifying the newly introduced dirty outdoor air, they’ll worsen the air quality in their homes instead of improving.

Contrarily, if you’re living in the US Los Angeles, you can safely introduce as much outdoor air as you like without purifying it because the outdoor air quality is very good with the level of PM2.5 at merely 2.5 μg/m3 (as of 17/3/22) which is 2 times better than the WHO long-term recommendation.

However, there are days in Los Angeles where the level of PM2.5 exceeds 5 μg/m3 but as long as the average level in a year is within 5 μg/m3, air purification is not necessary.

Forced Ventilation

Forced ventilation means introducing outdoor air by force or by means of a ventilation fan. There are two types of forced ventilation; a) through an air conditioner and b) an independent ventilation fan.

Introducing Outdoor Air via an Air Conditioner

Outdoor air can be ducted to certain types of air conditioning units such as ceiling cassette units, ceiling ducted units and central air handling units.

To see more details on the outdoor air duct connection, see my post How to Ventilate a Room with Air Conditioner?. For outdoor air duct design, see my post HVAC Ductwork Design Guide (Layout, Duct Size & CFM).

Introducing outdoor air via an air conditioner provides the benefit of air filtration. However, the air conditioner must be equipped with a HEPA filter to be effective. Therefore, dirty outdoor air can be purified and then, only the air quality in the house can be improved.

Introducing Outdoor Air using a Standalone Ventilation Fan

Outdoor air can also be introduced by means of a ventilation fan. However, the ventilation fan is standalone and not connected to any air conditioner. As you might have known already, this is not applicable if the outdoor air quality is poor.

If you’re living in a place where the outdoor air quality is good, introducing outdoor air using an independent ventilation fan ensures that your home has good air quality even when the air conditioner is not in operation.

However, both natural and forced ventilation will bring in warm/cold outdoor air which increases the power consumption of your heat pumps/air conditioners. In this case, you might want to consider using an energy recovery ventilator (ERV).

Introducing Outdoor Air using an Energy Recovery Ventilator

An energy recovery ventilator (ERV) uses the cold/warm exhaust air to cool/heat the newly introduced outdoor air and save up to 68% of energy depending on conditions compared to a system without an ERV.

Basically, an ERV is a heat exchanger. During the summer, it transfers the cold air that is being exhausted out of the house into the newly introduced outdoor air so that the air conditioner doesn’t need to consume too much power in order to cool the warm outdoor air.

Vice versa, during the winter, an ERV transfers the warm air that is being exhausted out of the house into the newly introduced outdoor air so that the space heater or heat pump doesn’t need to consume too much power in order to heat the cold outdoor air.

However, ERVs don’t have built-in fans. Often, they are installed in between fresh air ducts, relying on the airflow of air conditioning fans or ventilation fans.

Although forced ventilation with ERV introduces outdoor air to improve the air quality in homes while optimizing the additional power consumption for cooling/heating, it often still depends on the operation of air conditioners.

So, if you wish to improve air quality in your home but the outdoor air quality is poor and you don’t want to increase the energy usage for cooling/heating too much, what’s the option?

Fresh Air Unit with Energy Recover (New)

I got a chance to visit an HVACR exhibition held in Kuala Lumpur Convention Centre, Malaysia. From there, I found a new product that introduces outdoor air independently while having an energy recovery function to reduce energy use for cooling/heating.

In my opinion, the product fulfills all the technical requirements needed to improve air quality in homes.

First, it is an air purifier with a primary filter, a secondary filter and a HEPA filter. Second, it has an energy recovery heat exchanger built within to reduce the additional power consumption for cooling/heating the newly introduced outdoor air. Third, it is designed to introduce outdoor air independently.

Basic Working Principle of AstroCube from AAF

Not only that, its HEPA filter has antivirus and antibacterial properties that prevent the growth of microorganisms such as mold and fungus on the filter. Moreover, it has a display controller that shows the current air quality level with figures of PM2.5, PM10 and CO2.

I also learned that in order to have sufficient oxygen in the room, the CO2 level must be kept below 1000 ppm. Fortunately, the product is capable of displaying the CO2 level in real-time.

Products like this are suitable to be used by people living in places where the outdoor air quality is very bad so that they don’t have to open windows and doors to achieve the minimum air change per hour. Furthermore, they don’t have to worry that their air conditioners must be turned on in order to purify the air.

However, I suppose the cost of these products is not cheap. Nonetheless, they are still good products that fit the right people for the right applications.

Myth: Sterilize Air with UVGI

Many people had been misled by marketers to believe that UVGI can sterilize air. However, it is not entirely true. For deep understanding, see my post Do UV Lights in HVAC Work? (Check These Parameters).

UVGI stands for ultraviolet germicidal irradiation. Fundamentally, UVGI is able to inactivate viruses and bacteria. However, an amount of air or a surface must be exposed to UVGI for a prolonged period of time in order to effectively inactivate viruses and bacteria.

Basically, you need to circulate the same indoor air multiple times through a UVGI device in order to sterilize it. In reality, this is rarely achieved. However, surfaces sterilization is possible.

Some air purifiers have a UVGI system. However, the UVGI system is meant to prevent the growth of harmful microorganisms on their HEPA filter, not to sterilize the air and drastically improve air quality.

Conclusion

To improve air quality in homes, air purification with HEPA filters is inevitable. However, certain harmful pollutants are not able to be filtered even by HEPA filters. Hence, adequate air change must be done.

Introducing new outdoor air to replace old indoor air in homes can dilute and bring away harmful pollutants. However, without pre-filtration, such a method is not suitable for places with poor outdoor air quality.

So, regions of the world with poor outdoor air quality need to purify the outdoor air before introducing it to homes. Hence, introducing outdoor air via an air conditioner with HEPA filters or a dedicated fresh air purification unit is a viable option.

Nevertheless, the introduction of outdoor air will increase the power consumption for cooling/heating. For that, an energy recovery heat exchanger can be utilized.

In addition, filters with antibacterial or the use of UVGI on filters are additional features to prevent the growth of harmful microorganisms on filters.

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.


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Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

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