Health officials are urging people living near Awatoto or downwind of the factory to close windows and doors and to turn off air-conditioning units.
Medical Officer of Health Dr Nick Jones said smoke from the fire and the burning sulphur could affect people who had breathing issues like asthma or bronchitis and make their conditions worse.
Jones said anyone experiencing breathing difficulties should get medical help.
Hall said the fire was believed to have started in the conveyor belt at the top of the second shed.
"We are unsure what caused it at this stage but the product is susceptible to spontaneous ignition at low temperatures."
He said fire crews were only able to use low pressure deliveries to put out the fire, as there was a possibility of a dust explosion due to the large amounts of sulphur.
"As we add water there is more of a chance of the sulphur going in to a dust cloud but we have to use water to extinguish the fire so we are using spray rather than jets."
The fire service were on their "fifth alarm" so have 12 appliances and about 60 firemen fighting the blaze.
Hall did not know how long it would take for the fire to be resolved but expected to be there for a long time.
Rush hour traffic on between Hastings and Napier is backed up as a result of the fire.
Police said in a statement late this afternoon that traffic on Farndon Rd and State Highway 2 was moving very slowly.
"Traffic on Pakowhai Rd is down to walking pace. Please consider delaying your travel home or find an alternate route such as going via Taradale or Fernhill."
SH2 has since been closed from Waitangi Rd to Farndon Rd due to the hazardous event and to allow fire and police facilities immediate access.
The Napier-Hastings Expressway traffic is backed up so people should allow extra time for travel.
Police were also patrolling the cycleways to stop any cyclists filtering in to the surrounding area.
