Cleaning Outdoor Air for Improved IAQ

The United States Environmental Protection Agency uses six “criteria pollutants” as indicators of outdoor air quality, and has established for each of them a maximum concentration above which adverse effects on human health may occur. Of the six pollutants, ground-level ozone (O3, commonly called smog) has been suggested as being the most widespread health threat.

Although outdoor ozone is a major concern and there have been concerted efforts to reduce its concentration, this has proven difficult. Comparisons obtained with diverse data sets have shown an increase in tropospheric ozone levels of between 1 and 2% per year over the last 30 years. Indeed, “…there have been enormous increases in background ozone concentrations over the past half-century.” And projections are that this trend will continue. Ozone will continue to rise during the 21st century in less developed countries, as they burn more coal for energy, more people purchase and drive automobiles, and more goods are shipped by truck.

As of March 2017, the EPA identified part or all of 293 counties in 28 states and the District of Columbia as having air quality that violates the current 8-hour federal air standard for ozone. These areas encompass a total population of almost 133 million people or more than 45% of the population. Since ozone levels decrease significantly in the colder months in many areas, ozone is typically monitored only during the “ozone season,” typically May to September. During this period the U.S. nonattainment areas have covered as many as 455 counties in 31 states and almost 160 million people or more than 50% of the population.

Air monitoring data for ozone and other criteria pollutants along with air quality indices is available in many countries. It can be used to determine compliance with national air quality standards as well as determine the requirement or establish the need for outdoor air cleaning. U.S. partner agencies6 and international government agencies responsible for collecting and disseminating air quality data can be found on the AIRNow website, a cross-agency U.S. government website.


Ozone is not emitted directly into the air, but is formed by nitrogen oxides (NOx) and volatile organic compounds (VOCs) that in the presence of heat and sunlight react to form ozone. Ground-level ozone forms readily in the atmosphere, usually during hot weather. NOx is emitted from motor vehicles, power plants and other sources of combustion. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, and other industrial sources; dry cleaners, paint shops and other sources using solvents; consumer and commercial products. Changing weather patterns contribute to yearly differences in ozone concentrations from city to city. Also, ozone and the pollutants that cause ozone can be carried to an area from pollution sources located hundreds of miles upwind.

The reactivity of ozone causes health problems because it damages lung tissue, reduces lung function and sensitizes the lungs to other irritants. Ambient levels of ozone not only affect people with impaired respiratory systems, such as asthmatics, but healthy adults and children as well. Exposure to ozone for several hours at relatively low concentrations can significantly reduce lung function and induce respiratory inflammation in normal, healthy people during exercise. Other symptoms include chest pain, coughing, sneezing and pulmonary congestion.

Even with the acute health effects attributed to ozone, controversy remains regarding the relationship between ambient ozone and mortality worldwide. Although many studies have linked elevations in tropospheric ozone to adverse health outcomes, the effect of long-term exposure to ozone on air pollution–related mortality remains uncertain. However, as more data becomes available, definite links are being established. An 18-year study in the U.S. shows that people who live in areas with high ground ozone levels face a 30-per cent greater risk of death due to respiratory problems. A study of four years’ worth of data for Shanghai, China concluded that an independent association exists between ozone and total and cardiovascular mortality. It was further concluded that current levels of ozone had an adverse effect on the health of the general population. Others have reported an increase of 10 ppb outdoor ozone concentration has been linked to increased mortality rate of 0.87%.

Outdoor Ozone Standards

Criteria pollutants are those for which national ambient air quality standards have been developed and are those that can injure health, harm the environment, and cause property damage. Most lists include five pollutants along with ozone; carbon monoxide (CO), lead (Pb), nitrogen oxides (NOx), particulates (PM10 and PM2.5), and sulfur dioxide (SO2). Some lists also include volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Table 1 shows a comparison selected air quality standards and guidelines for ozone.

Table 1. International Ambient Air Quality Standards for Ozone (mg/m3).


Reported levels of concern for ozone exposure range from 0.1-0.25 mg/m3 (50-120 ppb) and inhalation intakes of indoor ozone are estimated to be between 25 and 60% of the total daily ozone intake.

In addition to direct exposure from ozone, there is also the potential for exposure to ozone reaction products: “reactions with ozone have the potential to be quite significant as sources of compounds that are often quite odorous and potentially damaging to both human health and materials.” Weschler estimated that the average daily indoor intakes of ozone oxidation products are roughly a third to two times the indoor inhalation intake of ozone itself. These oxidation products between ozone and (unsaturated) hydrocarbons can include such materials as formaldehyde, acrolein, hydroperoxides and fine and ultrafine (submicron) particles. Studies of ozone reactions with typical indoor surfaces reported byproducts, some of which are stable, such as organic acids and carbonyls. A multitude of unstable products and secondary organic aerosols can also form. Minimizing indoor ozone concentrations reduces the generation of these harmful byproducts, as well as ozone exposure.

Over the past few years several key studies have pointed to ambient ozone entrained into HVAC systems as a health risk factor to building occupants. Building related symptoms (BRS) are those that occupants experience while they are in a specific building but go away when they leave. A recently published statistical analysis of the U.S. EPA BASE study of 100 randomly selected large U.S. office buildings showed that ozone in the outdoor air was significantly associated with increased upper respiratory, dry eye, neurological, and headache BRS. Buildings with very low ozone in the outdoor air had, on average, up to 45% less prevalence of BRS. None of the 100 buildings in this study had extremely high ozone levels, and only one building was in an area that exceeded the current 8-hour federal air standard for ozone of 0.16 mg/m3 (75 ppb) – the median level was about 35 ppb.

Ozone and ASHRAE

ASHRAE Standard 62.1-2016

Ground level ozone is known to enter buildings through ventilation systems and through infiltration. It negatively impacts the indoor environment and its removal is required by some building standards. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 62.1-2016, “Ventilation for Acceptable Indoor Air Quality,” specifies minimum ventilation rates and IAQ that will be acceptable to the human occupants of a building. However, it acknowledges that acceptable IAQ may not be achieved in all buildings meeting the requirements of the standard – a prime example being a building located in a non-attainment area for one or more of the criteria pollutants listed in national air quality standards. Meeting minimum ventilation requirements in these instances only results in substituting one group of pollutants for another – those with sources outside the building for those internally generated – and increase the likelihood of BRS.

Perhaps because of what many deem as unsubstantiated objections due to a general lack of knowledge about air cleaning systems for gaseous contaminants in general and for ozone specifically, there has been a growing concern about simply ignoring the quality of the outdoor air and using it carte blanche for ventilation purposes. If the outdoor air, at the very minimum, does not meet national air quality standards it has been and continues to be strongly argued that this air must be treated before it is introduced into a building.

As a compromise to requiring air cleaning for all areas where outdoor air may be deemed “objectionable” or “unacceptable” and wanting to avert potential controversy over outdoor air cleaning requirements, ASHRAE Standing Standards Project Committee (SSPC) 62.1 approved a change to the standard that requires air cleaning requirements for ozone under certain conditions – this being where the most recent 3-year average annual fourth-highest daily maximum 8-hour average ozone concentration exceeds 0.107 ppm (2.09 mg/m3). While reducing the ozone concentration indoors may have a beneficial health effect; this requirement is primarily intended to reduce discomfort by reducing irritation due to ozone and its oxidation byproducts.

ASHRAE Standard 189.1

Whereas SSPC 62.1 was unsuccessful after several attempts to make air cleaning for ozone mandatory in all non-attainment areas even with the growing evidence of the harm ozone can cause in indoor environments, there were no further actions with regards to ozone until ASHRAE Standards Project Committee 189.1 began its work.

As one ASHRAE’s newest standards looking towards net-zero buildings and sustainability, Standard 189.1 “Standard for High-Performance Green Buildings Except Low-Rise Residential Buildings” was first published in 2009 with the purpose being to provide requirements for the siting, design, construction, and plan for operation of high performance, green buildings. It also provides criteria that address (among other things) indoor environmental quality (IEQ).

Provisions of Standard 189.1 address indoor air quality whereby the building must comply with specific sections of ASHRAE Standard 62.1 but with specific modifications and additions that made air cleaning for ozone mandatory under certain conditions. Specifically, those requirements provided in Standard 189.1 supersede the similar requirements in ASHRAE Standard 62.1.

Standard 189.1 specifies that “When the building is located in an area that is designated “non-attainment” with a national ambient air quality standards for ozone as determined by the authority having jurisdiction, (in the U.S., by the USEPA) air cleaners having a removal efficiency of no less than the one specified in ASHRAE Standard 62.1 Section shall be provided to clean outdoor air prior to its introduction to occupied spaces.” Standard 189.1 strengthens the requirements to investigate and address outdoor air quality that began with Standard 62.1 and mandates remedial actions for the benefit of building occupants.


Mandatory air cleaning for ozone is appropriate because of the substantial number of people living in non-attainment areas, and the negative impact ozone has on IAQ and occupant well being.
However, source control is most often neither feasible nor practical for outdoor air pollutants. Even if one could use ventilation (dilution) control, the use of large amounts of outdoor air is neither cost effective nor energy efficient. Therefore, air cleaning must be employed.

Conventional wisdom has held that because essentially every building HVAC system already has particulate filters, they should already be able to address the large majority of particulate contamination issues. However, when it comes to ozone, this provision of the Standard 62.1 has been almost universally ignored. The primary excuses being given are the high cost of installing, operating and maintaining the air cleaning equipment and that there is no standard method by which the performance of these systems can be evaluated. Even with these concerns, many are endorsing the use of air cleaning for ozone.

    Weschler states “Indoor exposure to ozone and its oxidation products can be reduced by filtering ozone from ventilation air and limiting the indoor use of products and materials whose emissions react with ozone. Such steps might be especially valuable in schools, hospitals, and day care centers in regions that routinely experience elevated outdoor ozone concentrations.”

    Apte believes “If the observed ozone-BRS associations are confirmed as causal, ventilation system ozone removal technologies could improve building occupant health when higher ambient ozone levels are present.

If one is required to apply air cleaning for compliance with ASHRAE standards 62.1 or 189.1, or any other standard or code for that matter, the next step is to evaluate and select the appropriate control technology. The most common air cleaning technology in use for outdoor air employs various adsorbent media incorporated as an integral part of an HVAC system. Properly designed, gas-phase air filtration systems can effectively reduce ozone and many other gaseous pollutants to well below regulatory levels and below those considered objectionable by most building occupants. These systems also have the potential for energy savings.20

The types and numbers of gaseous contaminants one would encounter in outdoor air in addition to ozone require that air cleaning systems need to be equipped with a minimum of two different Purafil media – Purakol brand granular activated carbon and Purafil SP. The preferred system would contain these media in two discreet filter beds.

It not always practical to incorporate a gas-phase air filtration system into an existing HVAC system. Retrofit applications in particular present challenges to the building engineer who is often limited by lack of physical space for the system, sufficient static pressure in the air handling system, or budgetary constraints. In these cases, panel filters employing adsorbent-loaded nonwoven media or newer extruded carbon composite structures can many times be placed in existing air handlers with little or no additional equipment or retrofit costs.

Air Cleaning Systems for Ozone and Outdoor Air

Air cleaning for ozone typically involves the use of a virgin activated carbon in granular form (GAC). The carbon essentially acts as a catalyst by serving as a site to concentrate the ozone and allow sufficient residence time for auto-oxidation. Ozone is also removed by surface oxidation on the carbon itself through a direct chemical reaction with the media. Air cleaning systems with a 1” (25 mm) bed of carbon can provide the required level of protection over an extended period. These filters may be applied as modules, trays, or “V-bank” configurations in either side access housings or front/rear access frames.

Purafil’s Purafilters employing an adsorbent-loaded non-woven fiber matrix or our PuraGRID filters employing GridBLOK extruded carbon composite media can be used as alternative to packed-beds of GAC. These provide much more flexibility in their application because they are available in all standard filter sizes. Many building owners use these filter types during the “ozone season” and then remove them for the remainder of the year. However, this practice oftentimes leads to permanent removal of these filters without annual replacement. Also, considering that many locations experience elevated levels of ozone well beyond the traditional May-September ozone season, it is recommended that any filters used for ozone control remain in place year-round. Purafilters as a true combination particulate + chemical filters are available that can replace existing particulate filters to provide for efficiency and economical control of both ozone and particulates.

Generally speaking, a properly designed, installed, and maintained gaseous air cleaning system will be able to remove significantly more than just ozone from the outdoor air. Even in non-attainment areas, the resulting indoor air quality will be improved to the point that building occupants will be able to tell when the media is spent and should be replaced, regardless of its efficiency against ozone.

Air Cleaning Case Study

An office building located in the southeastern United States was going “green” to attract and hold tenants. Part of this effort included the use of enhanced air cleaning for both indoor and outdoor air. The primary contaminants of concern in the outdoor air were ozone and volatile organic compounds (VOCs). Historically, ozone had averaged 60-100 μg/m3 (30-50 ppb) with peaks up to 300 μg/m3 (150 ppb) and VOC levels ranged from 80-150 μg/m3 with peaks as high as 210 μg/m3 during the months of May – September.

MERV 6 and MERV 11 particulate filters were already in use in building’s air handling equipment and there was no room for additional hardware to accommodate the use of V-bank modules, so 4” (100 mm) Purafilter combination particulate / chemical filters were recommended. These were accepted as replacements for the MERV 6 filters with conditions that a minimum 90-day filter life was achieved. If the Purafilters proved effective, meaning ≥50% removal for VOCs and ≥40% removal for ozone, they would be used on a continuing basis from April to September and then replaced with the MERV 6 filters from October to March.

Upstream and downstream ozone and VOC concentrations were measured nearly daily from May to September of 2007 to gauge the effectiveness (efficiency) of the Purafilters. At the end of 90 days the VOC efficiency had dropped to ~45%, but the ozone removal was still above 95% (Figure 1). This convinced the owner that these combination filters were effective and their use would result in improved IAQ. It was felt that the very high effectiveness for the Purafilters against ozone – even as the effectiveness for VOCs approached zero – meant that the potential for adverse respiratory health effects due to ozone would be significantly reduced or eliminated. Also, the formation of new chemical species due to reactions between VOCs and ozone, many of which would be considered highly irritating, would be similarly reduced or eliminated.

Figure 1. Purafilter performance against ozone and VOCs (outdoor air, urban location).


Typical air cleaning systems employing granular adsorbents in modules or tray have an estimated annual cost of US$0.20 – 0.40 per ft2 per year.22 The amount of media contained in these systems makes them attractive due to increased intervals between media replacement. However, there may have substantial installation costs due to additional hardware required.

Reducing these annual O&M costs and avoiding the front-end costs associated with these systems can be achieved using 4” (100 mm) Purafilters or 2” (50 mm) PuraGRID filters described above. These would add an estimated US$0.07 – 0.10 and US$0.04 – 0.05, respectively, per ft2 per year to HVAC operating costs.


Over the past few years several key studies have pointed to ambient ozone introduced into HVAC systems as a health risk factor to building occupants. Ozone in the outdoor air has been associated with an increase in building-related symptoms (BRS) and buildings with very low ozone in the outdoor air had up to 45 percent less prevalence of these same BRS.

In contrast to particle filters, gaseous air cleaners are used in only a small minority of buildings because of a lack of strong and enforceable requirements for cleaning the outdoor air, the associated higher operating and maintenance costs and a general lack of perceived benefits in doing so. Regardless, new products for gaseous air cleaning are being developed.

A combination filter has been developed, Purafilter, that provides the ability to effectively and economically control both particulate and gaseous contaminants for the improvement and maintenance of IAQ. A study has shown that even when the filter is effectively spent with regards to other gaseous contaminants, ozone removal efficiency can still be well above 50%.

Another innovative technology employing an extruded carbon composite medium, PuraGRID, has the potential to provide better even better performance and economics for the control of ozone.

Most ventilation standards and codes make no distinction between particulate and gaseous contaminants when considering outdoor air quality and its impact on IAQ. Nor do they distinguish between these contaminant types in indoor air except in the methods used for their control. Although requiring the use of air cleaning systems for ozone is an important first step in addressing the quality of the outdoor air being used for ventilation purposes, many still feel it just that – only the first step. A change proposed to ASHRAE Standard 62.1 would require ozone air cleaning in all areas of non-attainment which many feel is only inevitable given the current level of understanding regarding ozone and its impact on IAQ and health.

That acceptable IAQ may not be achieved in all buildings meeting minimum ventilation requirements, means we must also acknowledge the fact that although adding a requirement for ozone control will bring us a closer to our goal, we may still have a considerable way to go to assure that meeting the requirements of any ventilation / IAQ standard or code will indeed assure and provide “acceptable indoor air quality.”