New research indicates some guidelines for minimum ventilation periods after phosphine fumigation are inadequate for meeting industry standards for phosphine levels upon delivery to receval sites.

A series of silo-scale trials led by Dr Manoj Nayak at the Queensland Department of Agriculture and Fisheries (DAF), supported by GRDC, tested phosphine label advice for fumigating and venting wheat.

These involved 10 days of fumigation at the label rate of 1.5 grams per metre cubed followed by:

• two hours on, two hours off fan-forced ventilation for one day;
• five days of passive ventilation (no fan); and
• constant ventilation (fan on) for one day.

It showed that running a fan for two hours on and two hours off over a day would be a potential “disaster” for growers delivering grain to receival stations, Dr Nayak says.

Under workplace health and safety standards, Australian grain must not exceed phosphine levels of a threshold limit value–time weighted average (TLV-TWA) of 0.3 parts per million (ppm). This TLV-TWA threshold is based on an acceptable phosphine exposure level for someone working eight hours a day over a five-day working week.

On 1 December 2026, the TLV-TWA will be reduced from 0.3ppm to 0.05ppm, which will require significant changes to venting if industry standards for clearance testing are not updated.

Testing times

The DAF testing of the two-hour interval venting method found concentrations under  the accepted limit of 0.3ppm on the silo surface. But spear tests to a depth of one metre, such as those used at some receival sites, showed levels of phosphine concentration above the 0.3ppm limit up to eight days after ventilation.

“We believe this (two hours on, two hours off) was a recommendation based on energy saving, but according to our test it should  not be relied upon,” Dr Nayak says.

A second trial in the same-sized 35-tonne silos with wheat at the same 24°C temperature showed passive ventilation (for five days), with the top of the silo lid completely open and  the bottom unsealed, was also inadequate.

Spear tests returned phosphine concentrations of up to 20ppm during the passive ventilation period in the silo. Concentrations dropped to acceptable levels a day after being loaded into a truck, but increased again above the threshold (to more than 0.5ppm) four days later.

“Our trials show that this grain would have been rejected at delivery sites that use a spear sample clearance of 0.3ppm, if delivered two to four days after outload,” Dr Nayak says.

So even if it is on the label, I would be very seriously thinking of not using these ventilation methods if I was sending grain out.

The trials showed, however, that venting was effective when fan-forced airflow ran consistently. In an eight-tonne silo, with wheat stored at 23°C, concentrations of phosphine were below 0.3ppm in both surface and spear tests after constant fan-forced ventilation for one day.

“I think that the key is to have an aeration system that can push the gas out regularly and continuously,” Dr Nayak says.

To the limit

GRDC Grain Storage Extension national coordinator Chris Warrick says the research is aiming to help avoid loads being rejected while maintaining a safe workplace.

He says a common reason grain is rejected is because lower phosphine levels are recorded in on-farm surface testing compared with the readings from spear tests at receival sites.

The DAF trials demonstrate this discrepancy. The surest way to ensure grain will meet the required standard is to spear test on-farm,  Mr Warrick says.

“There are so many variables that can impact phosphine levels – grain type, storage size, temperature and how long grain is locked up after fumigation before ventilation. So our advice is to use a low-range phosphine meter and spear test to check for phosphine before out-loading.”

Research has also demonstrated grain desorbing phosphine after it has been moved or increased in temperature, Mr Warrick says. “So if trucks are loaded the day before delivery, it’s worth spear testing in the truck to check for phosphine just before leaving the farm.”

The industry is working to develop testing protocols that take into account the differences between phosphine spear test readings and what operators could be exposed to. “We are looking to a new standard for clearance because at 0.05ppm a spear test would possibly be unworkable,” Mr Warrick says.