Salvesan bus disinfection project

Aim  

The aim of the project was to assess the effect of Salvesan hypochlorous acid delivered via an automated fogging system on the micro bioburden across a range or surfaces. The misting system was deployed three times in total with swabs taken after each application. This was to look at the additive effect of misting on bacteria numbers.

Methodology

A bus with a new automated fogging system installed was made available at a large UK city bus dept. Swabs were taken across a range of hand touch surfaces on board the bus before misting. Stencils were used to ensure that swabbing area was consistent and repeatable. All swabs were taken by the same person using the same technique. Nine areas were chosen, as outlined below:

One set of pre treatment swabs were taken then the bus doors were closed and the first 20 second misting cycle was deployed. A three minute dwell time was left between end of the misting cycle ad taking the next set of swabs. The second set of swabs were taken in all 9 areas. This was repeated again after a second misting (total time 40 seconds) and a third misting (total 60 seconds mist exposure).

A total of 36 swabs were taken: 4 swabs per site. Swabs were couriered to the lab (One Scientific, Avonmouth) where they were analysed for Aerobic Colony Counts using the pour plate method.

Results

All 36 swabs were successfully analysed. Results are reported as colony forming units per swabs (cfu/swab) and are presented in the following table:

The results indicate that the initial loading on surfaces is variable with most counts at an acceptable standard (counts of <100 cfu/swab in sample areas of this size are considered acceptable). It is not known when the bus was last used, when it was last cleaned or how many mist cycles had been run on the bus prior to the testing. It would be interesting to look at levels of contamination on these surfaces are on buses in service as the very low counts seen could be due to recent cleaning / previous mist applications.

The total number of bacteria recovered in the pre-treatment samples was 15,400 which was reduced to 70 after 3 misting cycles (99.5% reduction). The most contaminated surfaces were the fabric seats and the pillar behind the driver. The hard surface with initial contamination (pillar behind driver) was reduced to acceptable levels after one mist cycle. One of the fabric seats was also decontaminated after one misting cycle. However the other seat had something on it that took three cycles to remove (each cycle removed 1 log of bacteria). Fabric / soft furnishings are difficult to clean so this is still a good result considering the initial high loading observed. However these averages are skewed by the dataset for the heavily contaminated seat. If this data set is removed then the total number of bacteria recovered initially was 2,400 cfu which was reduced to 30 cfu after just one 20 second misting cycle.

Conclusions

On average each misting cycle removed 1 log of contamination (90% removal after one mist, 99% after two mists, 99.9% after three misting cycles).