Designing HVACs: What should you know about indoor air quality?

Energy efficiency has become the primary focus of HVAC design.

While that’s a good thing, it’s equally important that your design consider the health and comfort of the end users. According to the EPA indoor air quality is, on average, two to five times more polluted than outdoor air. In fact, the Environmental Protection Agency estimates that 33% to 50% of commercial buildings in the U.S. have poor indoor air quality levels.

Poorly designed HVAC systems can harm air quality. Another way HVACs can adversely affect indoor environments is through the lack of maintenance. Here again, design often plays a part, making it difficult for personnel to service the equipment. You can learn more in Why you need to design HVACs for regular maintenance.

HVAC can spread dust, bacteria, allergens or other harmful substances. Depending on the type of HVAC, it can be the source of bringing outdoor pollution inside. If it heats or cools the building excessively, then the occupants experience discomfort. Humidity can be another problem with HVACs. An immoderate amount of moisture or dry air also causes discomfort. Moisture has the added problem of being the source of mold.

For a building’s indoor air environment to be healthy, it needs to provide:

• Adequate ventilation – introducing and distributing clean air
• Controlled contaminants
• Comfortable temperature and humidity levels

While some particulate matter comes from within a building, an HVAC system can draw in outdoor air and with it, pollutants, to supply the building. The most common pollutants are:

• Volatile organic compounds (VOCs)

These are organic chemicals emitted as gases from products or processes, such as cleaning agents and air fresheners. Coming from outside, sources of VOC contamination might include chemicals in polluted groundwater, brought inside during water use. The Threshold Limit Value (TLV) is the limit a person can be exposed to a certain VOC without suffering adverse effects. This is measured in ppm (parts per million). Below are VOC indoor air-quality limits.

Common VOC levels ppm around the house

VOC	Common in	TLV
Ethanol	Cleaning agents	1,000 ppm
Formaldehyde	Moulded plastics, plastic finishes	0.1 – 0.3 ppm
Acetone	Furniture polish, nail polish, wallpaper	750 – 1000 ppm
Benzene	Glue or paint applied to furniture	0.1 ppm
Dichlorobenzene	Mothballs, deodorant	25 – 50 ppm

• Carbon monoxide (CO)—Part of the danger of carbon monoxide is that you can’t smell, taste or see it. Varying levels on CO and their effect are shown below, including air quality norms.

Air-quality CO ppm

TLV	Consequences
250 – 400 ppm	Normal background concentration in outdoor air
400 – 1,000 ppm	Concentrations typical of occupied indoor spaces with good air exchange
1,000 –2,000 ppm	Complaints of drowsiness and poor air
2,000 – 5,000 ppm	Headaches, sleepiness and stagnant air. Also typical: poor concentration, loss of attention, increased heart rate and slight nausea
5,000 ppm	Workplace exposure limit (8-hours) in most areas
>40,000 ppm	Exposure may lead to serious oxygen deprivation resulting in permanent brain damage, coma, or death

• Particulate matter (PM)

PM is a mixture of both solid particles and liquid droplets that hang in the air. Examples include dust, pollen, soot and smoke. The particles vary in size, but it gets especially dangerous when you’re dealing with indoor PM10 levels and below. This is the smaller particulate matter 10 micrometres in diameter or smaller which can be inhaled. The PM2.5 threshold is even worse. This is particulate matter of 2.5 micrometres or less in diameter. It can penetrate the lung barrier and enter the blood system, causing serious health issues, such as cardiovascular and respiratory disease and cancers, even at extremely low concentrations.

Indoor air quality guidelines and standards

There are no hard and fast standards for indoor air quality (IAQ), but the consensus recognizes guidelines offered by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

ASHRAE air-quality standards are the most widely used guidelines in the U.S. Their guidelines don’t set standards for the design of HVACs, but instead define minimum values and acceptable performance within the HVAC industry. For example, ASHRAE Standards 62.1 and 62.2, which are used by CIBSE, define “roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality in residential buildings.”

They specify minimum ventilation rates and other measures with the purpose of reducing a building’s negative impact on health. Building owners and managers can use the ASHRAE 62.1 ventilation spreadsheet to calculate the ventilation rate of current systems that serve multiple zones.

You should incorporate the ASHRAE recommended ventilation rates into how you want your HVAC to perform, which helps you determine minimum fresh air requirements (Ashrae).

Site	Air-change rate standard: ASHRAE, per hour
Home	0.35 – 1
Hotel rooms	1 – 2
Offices	2 – 3
Retail shops	2 – 3
Schools, except for lecture halls	5 – 6
Sports facilities	4 – 8
Restaurants	6 – 8

The EPA recommends using ASHRAE Standard 62.1 for HVAC system verification. Testing is done to verify the HVAC’s performance prior to EPA air-quality testing, including space temperature and space humidity uniformity, outside air quantity, filter installation, drain pan operation, and any obvious contamination sources. The EPA also provides air-quality design values which are published annually to provide air-quality status of a given location relative to the level of the National Ambient Air Quality Standards (NAAQS).

Some companies test and issue EPA-recognized indoor air-quality certificates for buildings, which is a great selling point for building owners or managers to attract tenants or buyers. Green buildings must contribute to the well being of occupants in the form of air quality.

Another relevant body is the Occupational Safety and Health Administration (OSHA). There are no OSHA air-quality standards, however. OSHA is a regulatory body of the U.S. Department of Labor, tasked with ensuring safe working places. They provide guidelines that address common workplace complaints about IAQ, namely temperature, humidity, lack of outside air ventilation, and smoking.

In regards to OSHA air quality in buildings, there is Standard 1910.94 – Ventilation. It doesn’t address HVACs, but it covers air ventilation requirements on equipment for workers, such as those undertaking blast cleaning.

Indoor air-quality parameters

What is acceptable indoor air quality? The EPA, ASHRAE and other organizations pretty much agree on the parameters, listed here.

Characteristic	Conditions to be met
Particulate Matter (PM)	10 micrometers or less in diameter: 50 ug/m3; 2.5 micrometers or less in diameter: 15 ug/m3
Carbon monoxide	Less than 9 ppm
VOCs	Less than 500 ug/m3
Formaldehyde	Less than 27 ppb
Carbon Dioxide	ASHRAE CO2 levels are about 700 ppm above outdoor air levels (usually about 1,000 to 1200 ppm)
Humidity	Below 60%, ideally between 30% and 50% (EPA)
Temperature	68.5°F to 74°F (winter); 75°F to 80.5°F (summer) (ASHRAE)

How to measure indoor air quality

You can measure air quality in offices and other buildings using IoT sensors placed in various places.

The most accurate indoor air quality monitoring method, IoT sensors use a scale to translate the concentration of molecules of different particulates and substances. You’re alerted when anomalies are detected, enabling you to evacuate an area if pollution or humidity levels reach dangerous levels.

Air filters for the best air quality

A well-designed HVAC system can help improve IAQ levels. Air filters should be carefully considered. These use a rating system called Minimum Efficiency Reporting Value, or MERV.

The rating system is defined in ASHRAE Standard 52.2, which provides the testing conditions and the performance necessary to meet each MERV level. The higher the MERV rating, the more particulates are removed from the air. When you hear, for example, MERV 13 PM2.5, we’re talking about a filter that removes a minimum of 90% of PM10, a minimum of 85% of PM2.5, and a minimum of 50% of PM1.0. Below highlights the rating system.

MERV-rated air filters

	Minimum % of particles filtered
	PM2.5 zone
MERV rating	0.3 – 1.0 microns	1.0 – 3.0 microns	3.0 – 10.0 microns
1	-	-	>20%
2	-	-	>20%
3	-	-	>20%
4	-	-	>20%
5	-	-	>20%
6	-	-	>35%
7	-	-	>50%
8	-	>20%	>70%
9	-	>35%	>75%
10	-	>50%	>80%
11	>20%	>65%	>85%
12	>35%	>80%	>90%
13	>50%	>85%	>90%
14	>75%	>90%	>95%
15	>85%	>90%	>95%
16	>95%	>95%	>95%

 

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