With the rapid urbanisation that India is witnessing, there are more and more buildings being constructed and occupied in large cities all across the country. This means there are more cars on the road, there is more construction activity in the vicinity of these buildings, there are more industries supporting these buildings. All this leads to a greater impact on the air quality around these buildings and we are now witnessing poor Air Quality Index (AQI) days as the normal, with visuals of city inhabitants moving around in thick smog, with masks for protection. While pollution of the environment around is a fact, it is also imperative to have these large buildings to house the huge number of people who come to work every day and help push India’s GDP to one of the highest in the world. Thus, while there is a problem of air quality and long-term solutions are being addressed by the various stakeholders, there is also a need to find a short term, local solutions to see that the occupants of buildings are working in a safe and habitable space.

Multi-tenant buildings pose a challenge to providing good quality air to the occupants as the Air Handling Units (AHU) are managed by the developer while the ducting and internal air circulation is in the control of the occupants or tenants. A good HVAC system, with the appropriate ventilation design and collaboration between the occupants and the landlord will enable the Indoor Air quality (IAQ) to be of the desired standards. This article describes a case study where the land lord and the occupant in the building collaborated to provide the occupants with a very high standard of indoor air through a retrofit project in an operational building.

Air quality requirements for buildings

ASHRAE Standard 62.1-2013 – Ventilation for Acceptable Indoor Air Quality is the widely used reference document for ventilation is buildings. The standard defines and lays downs various processes and parameters that ensure the minimum acceptable levels of air quality in the work space. The standard primarily addresses the design considerations. In addition to adhering to a good design, best practices in operation of the ventilation system as well as workplace hygiene help to improve the air quality.

The outdoor air quality is measured using the AQI scale. Eight parameters are measured and there are six categories of air defined in the AQI scale used in India. Figure 1 lists the AQI scale and the air constituents that are measured and their values for each category.

While the current focus in the large cities is around Particulate Matter (PM) 2.5 and PM 10 values, the other pollutants are equally harmful to the human body. The ASHRAE standards take a reference to external levels of certain pollutants such as CO2 and hence, the indoor air quality is a function of the external air quality.

The air quality challenge

The building where this retrofit was undertaken was in the NCR region and the ambient air quality had deteriorated to a very large extent. Heavy construction around the building, increased traffic on the roads, pollutants carried over in the air from neighbouring states due to the stubble burning by farmers were some of the reasons for the high AQI. While the landlord had provisioned for the best in class air filtration systems and ventilation design when the building was built four years back, these systems were not designed to cater for the level of pollution that the city was witnessing. The landlord was keen to provide the required air quality for a grade A development and a few of the tenants were also keen to provide their employees with a good and safe work environment. Thus, the developer undertook a retrofit project to improve the air quality in the building.

Building overview

The building ventilation consists of AHUs on each floor. The air to the AHU is via Treated Fresh Air (TFA) units in the terrace to cater for the 5 – 10 per cent fresh air requirement to maintain the IAQ as per the ASHRAE Standard. The AHUs and TFA units are controlled by a building management system and the fresh air openings are controlled by CO2 sensors. The AHU has a flow rate of 22000 CFM and MERV 8 and 13 filters are placed in the AHU in the existing arrangement.

Retrofit solution

Air quality assessment

To get an assessment of the quality of air that was being delivered by the existing system, air quality sensors were installed at pre-selected points in two floors. These systems enabled continuous monitoring of the air quality to see the trend of the air quality variation over the period of the day as well as over the weeks. The implementation team also studied the foot falls and usage patterns of the occupants, the openings in the building and the two floors where air could leak into from outside. Another aspect that was assessed was the quality of the existing air filtration system and the condition of the filters in the AHU and TFA.

Proposed solution

Based on the study undertaken by the building O&M team and the air quality assessment team, the most optimal solution arrived at was to install an electrostatic based air filtration system. The filter consists of specialised panels that are fitted in place of the existing filters on the AHU and works on the principle of ionisation of the air which breaks down the pollutants in the air. The pollutants are collected on collector plates and clean air passes through to the occupied space.  Figure 2 shows the working principle of an electrostatic filter.

Figure 2: Electrostatic Air Purification

Benefits to retrofit

Post installation, the air quality was monitored for a period of three weeks using the same continuous monitoring sensors used during the assessment phase. The data was analysed and normalisation carried out in case of any abnormal values on account of sudden changes in external or internal conditions such as a dust storm or a significantly higher footfall. The external AQI was also monitored to ascertain the improvement in the air quality inside the building. The system performed as per the expectations and there was significant improvement of the air quality at the AHU outlet. The improvement was on all the parameters, but more significantly, the PM 2.5 values were seen to be continuously below the WHO guild lines thus validating the installation of the air purification system. In addition to the sensor data, the feedback from the occupants was also taken and there was a perceptive improvement in the air quality as per the inputs received from the sample occupants queried.

No retrofit project passes scrutiny unless the benefits are clearly identified by the users. In this case, there were multiple benefits as follows:

–    Air Quality improvement: With outside PM 2.5 in the range of 300 – 350 (very poor AQI), the PM 2.5 levels before fitting the system were in the range of 150 – 180 which were above the desired values. With the retrofit, the PM 2.5 levels achieved were consistently below 60 and mostly in the range of 15-30.  This purification approach reduces lower value PM’s as well and equivalent to a MERV rating of 14. In addition to lower 2.5 lower levels, the filtration system also brought other pollutants including biological count and VOCs.

–    Energy Savings:      The typical media filters on AHUs cause a pressure drop across the screen resulting in higher energy usage. Since the retrofit system is based on ionisation, the pressure drop is significantly lower which results in lowering the energy usage. In this particular case, the energy consumption was reduced by 25-30 per cent. Taking into consideration that AHUs run for 12 hours a day, this leads to a substantial savings for the occupants.


Deterioration in the outdoor air quality is a major environmental issue and is being addressed by the various stakeholders in the fight against pollution. However, there is a crisis at hand and it is impacting the health of the occupants of buildings as they spend a large amount of the time in their workplace. Indoor air is more polluted than the outside air, especially if not treated or the fresh air quality injected to the system is inadequate. Thus, building owners and occupiers should look at ways to treat the outdoors as well as the recirculating air. While there is a cost involved in retrofitting these purification systems, the benefits far outweigh the expense. Employee turnover, loss in productivity due to respiratory problems, absenteeism is some of the hidden costs of poor air quality. While new constructions have purification systems built in, older buildings require an upgrade in the air purification system. The benefits will accrue to both the landlords through higher retention and rentals as well as to the occupants through a healthier workforce.