Importance of Air Particle Counts in Hospital Infection Control: Insights From a Cancer Center in Eastern India

Clean rooms are classified in a variety of different ways, which include the International Organization for Standardization classification (ISO standards 14644–1; classes 1–9), Federal Standards (FED 209E) having Imperial type classification (Class 1 to 100,000), and the Metric classification (Metric 1–7). ^{1 , 2} The Imperial Class 100 room correlates with ISO 2 and the Metric 3.5 standard and is based on airborne particle counts (APCs) of 0.5 μm/ft³. One hundred particles of 0.5 μm dimension per cubic foot by Imperial Standards equals 3,530 particles per cubic meter. ^{1 , 2} Maintaining the quality of air in critical areas such as bone marrow transplant or stem cell transplant units, clean operating rooms, and biological safety cabinets and laminar air flow hoods is essential for maintaining standards and optimizing outcomes for the patients and staff of the hospital.

In this study we describe the importance of APCs in maintaining and monitoring air quality in a cancer center in eastern India. The methodology included monitoring air quality using a handheld air particle counter (ErgoTouch Pro; Biotest [now MerckMillipore]). ³ This equipment measures airborne particles of 6 sizes (0.3 μm, 0.5 μm, 1 μm, 3 μm, 5 μm, and 10 μm) using lasers. It samples 0.1 ft³ of air in a single sampling time of 1 minute. The results can be reviewed with the time of the exact sampling and show both differential counts (each size) and cumulative counts (of all 6 sizes). The air particle counter gives real-time data within a minute, which is not the case with air microbial sampling or settle plate methods, which need 48 hours for bacteria and 5 days for filamentous fungi for enumeration of colony counts. ⁴

In a biological safety cabinet that is working optimally, APCs of all 6 sizes (0.3 μm to 10 μm) should be zero/ft³—both differential and cumulative.

In a high efficiency particulate air (HEPA)–filtered Class 100 operating room, during nonoperating hours, with a functional air-handling unit and optimal sufficient air changes per hour, the APCs of 0.5 μm particles are ideally less than 100 per ft³. The Report of the Joint Working Party on Ventilation in Operating Suites advised that all operating theaters should ideally have a ventilation equivalent of 20 air changes per hour. ⁵ Air changes per hour are calculated by dividing air supply rate by room volume.

Each set of readings performed on a particular day also details the minimum, average, and maximum reading for each channel, along with the standard deviation and standard error of these findings. For example, on a given day in February 2015 in the 8 operating rooms of this center, the 0.5 μm counts were as follows: minimum, 8/ft³; maximum, 279/ft³; average, 104.6/ft³; standard deviation, 80.19/ft³; and standard error, 20.71/ft³. Similarly, on a given day in February 2015 in the 8 stem cell transplant rooms of this center, the 0.5 μm counts were as follows: minimum, 80/ft³; maximum, 3,447/ft³; average, 1,320.35/ft³; standard deviation, 1,218.5/ft³; and standard error, 295.53/ft³ (see Online Table 1). For the stem cell transplant unit, the recommended air quality for its protective environment calls for at least 12 air changes per hour, central or point-of-use HEPA filters (99.97% efficiency in removal of particles ≥0.3 μm diameter), positive pressure differential of at least 2.5 pascals, ⁶ and HEPA-filtered environment in the entire complex, with spare sets of coarse and fine filters. It is much more difficult to maintain optimal APCs in stem cell transplant rooms because of the presence of patients, caregivers, staff, and furniture. Also, generally not all surfaces are epoxy coated, and a smaller number of HEPA filters results in fewer air changes per hour than in operating rooms.

Factors affecting APCs in a given environment are cleanliness of the area, air changes per hour, and integrity of the filter units in the air-handling unit. Any abnormality in the APCs should trigger a review of cleaning practices, air velocity checks using anemometers, and frequency of air-handling unit maintenance, including cleaning and integrity of coarse and fine filters as well as the HEPA filters. The latter may be assessed using the dioctyl phthalate test. Because of concern that dioctyl phthalate may have carcinogenic properties, it has been replaced by an alternative product. Polyalphaolefin is a noncarcinogenic liquid commonly used as a replacement for dioctyl phthalate. ⁷

It is expected that those premises within the regulated confines of a hospital environment that have low air particle counts would demonstrate low suspended microbial (bacterial and fungal) counts. But there are studies to suggest that the two may not always correlate. ⁸ The lack of correlation could be due to multiple reasons, such as calibration of instruments, quality control of culture media, inappropriate sampling, and nature of the particles (cultivable/ noncultivable). In all cases of air quality evaluation in clean areas, the physical inspection of premises must be performed so that cleanliness, infection control practices, engineering problems, and general maintenance issues can be verified, and corrective measures implemented, even before actual air quality checks are instituted.

In our practice, APCs are a primary surveillance technique for air quality monitoring, and microbial counts of air quality have been performed only when abnormal air particle counts are registered or specific pathogens have been suspected. This strategy has saved valuable time and resources and also avoided the difficulty of interpreting microbial colony counts in situations where standards are not always available or unambiguous.