A microbiologist with more than 40 years’ experience says the term “deep clean” thrown around by businesses is a “public pacifier” and firms are not doing enough to mitigate Covid-19 transmission risks following positive case exposure.

Ben Harris, medical microbiology scientist and lecturer at the University of Otago, says the now-famous phrase “deep clean” is in fact not deep, and research shows that the virus can continue to live on surfaces even after a clean has taken place.

Coronavirus and new variants have become 50 to 70 per cent more infectious, meaning much lower exposure to the virus can cause infection.

Harris says a “place of interest” or business premises could be infectious even two hours after Covid-19 exposure, depending on the flow of fresh air.

Manual cleaning, even with disinfectants, is an ineffective method to mitigate transmission risks, he says.

“The biggest risk is shared air, and then touch surfaces,” says Harris.

“I always use the analogy of a smoker, where smoke is the surrogate for the virus – the nearer you are to a smoker, the more smoke you breathe in, thus the more virus, and if you are at a far side of a room the smoke will still get there but it takes longer to breathe in.”

Harris remains cautious on just how thoroughly cleaned shops and public venues are after they have been shut for a so-called “deep clean”.

“You’re not going to get people going through and wiping down every bottle. What they do try and do from what I understand is use CCTV to see what [Covid-positive visitor] has touched and anything they have touched wipe down. Other than that, if air particles have landed on bottles that they haven’t touched, it is impractical to wipe down everything in store.

“With these ‘deep cleans’, [businesses] somewhat rely on time – with time the virus will [reduce] on a surface.”

On hard surfaces such as woods and plastics, the virus can survive up to approximately eight days, and up to three days on soft surfaces such as clothing, he says.

Lifts are also big incubators of infection as the air is retained.

“With time there’s a degree of reliance on hoping the virus will die, but there’s also the business pressure on needing to get patrons back in again.”

So what do businesses actually need to do to get rid of any traces of Covid?

Harris’ first piece of advice is “not to do a manual clean”, and instead use fog canon disinfectants as a way to effectively sterilise both the air and surfaces.

“If there is a known case [that has visited the premise] everything should be fogged with high-level disinfectant fog, not residual ones.”

That is all that is required, he says, adding that fog machines have the ability to completely sterilise an N95 valved mask, according to studies from Duke University.

Harris says he did not understand why businesses were not using fog canons as part of their deep-clean processes. He said they were far cheaper to use compared to manual labour, once the initial investment in the machines had been made.

How businesses are deep cleaning

The Herald understand shopping centre operators Kiwi Property and Scentre Group actively use fog canons to deep clean their premises, along with retirement village operator Ryman Healthcare.

Supermarket giant Countdown says it does not use fog canons and is satisfied with its current cleaning processes.

Kiri Hannifin, Countdown’s general manager of health, safety and wellbeing, said when alerted to a positive Covid case visiting one of its stores the business “proactively chooses to go over and above” the comprehensive cleaning and sanitising it does throughout a regular day.

“We have a detailed response plan in place for when we are notified of such a visit and part of that includes closing our store as soon as possible and undergoing a deep clean.

“The clean will cover all high touch areas including checkouts, self-checkouts, handles, trolleys, baskets, toilets and more. We always take advice from the regional health department on where the positive case may have visited throughout the store and make sure to deep clean any of those touchpoints too,” Hannifin said.

“We have had extra cleaning and sanitising in place since March 2020, and for us, this focus hasn’t changed since we were first on alert around the presence of Covid-19 in New Zealand.”

Supermarket co-operative Foodstuffs, which operates Pak’nSave, New World, Gilmours and Four Square chains, among others, said the guidelines for deep-cleaning are set by the Ministry of Health and involve cleaning and sanitising all store surfaces that are likely to have been touched or stepped on, including, walls, cabinets, handles and shelves.

“The deep-cleaning process is conducted in a specific order to ensure best practice results and the reduction of any potential cross-contamination. For instance, the cleaning starts with high up surfaces with the floors cleaned last and the cleaning agent used must be a 2-in-1 hospital-grade disinfectant and cleaner. Deep cleans are carried out by professional cleaning contractors and strict hygiene protocols are followed throughout the process,” said Antoinette Laird, head of corporate affairs at Foodstuffs.

Bunnings, who was recently at the centre of positive covid case visits, said in instances of Covid-19 exposure, it deploys commercial cleaners perform a deep clean of the store using antiviral disinfectants.

Kmart has also been contacted for comment.

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