The military has been called in to help battle a huge moorlands fire which has been raging for three days.
The blaze covers 3.7miles (6km) of moorland above Stalybridge and prompted the evacuation of up to 100 homes.
Greater Manchester Fire and Rescue Service (GMFRS) confirmed it had asked for military assistance.
One resident described seeing "ash falling like rain" and another said it "looked like the apocalypse".
Four schools have closed and a GP said he had seen patients with nosebleeds, coughs and eye problems caused by the smoke.
Firefighters are having to haul pumps and hoses for up to two miles in blistering heat to reach parts of the fire which has devastated swathes of land near Saddleworth Moor.
Prime Minister Theresa May said the government is keeping the situation "under constant review".
Greater Manchester's assistant chief fire officer Dave Keelan said a "request" had gone in for military assistance.
"That's for help to move high-volume pumps and to transport personnel to remote locations," he said.
Pressed on the arrangements, he said the military was likely to use Chinook helicopters to move two water pumps.
A Ministry of Defence spokeswoman confirmed the fire service had approached them "as a priority" and the MoD was "looking into what might be done."
Greater Manchester mayor Andy Burnham started a question time event in Stretford saying that if the Home Office agreed, military resources should be available by Thursday morning.
Andy has started by talking about the fire @manchesterfire are dealing with on Saddleworth Moor: "This afternoon a submission was made to the home office requesting assistance from the military. If the home office agree, resources will be available from first thing tomorrow."— Mayor Andy Burnham (@MayorofGM) June 27, 2018
United Utilities has also provided a helicopter to drop water on to hard-to-access areas of the fire.
Experts warned high levels of pollutants generated from the blaze could have a significant effect on people's health.
Emergency services have been giving out face masks to their own staff, residents and members of the media.
Stalybridge GP Richard Bircher said he had seen patients with nosebleeds, coughs and sore eyes caused by the smoke.
Hugh Coe, professor of atmospheric composition at the University of Manchester, said plume peak concentrations were "very high" and air quality close to the fire was "very poor".
He said pollution plumes had been detected in the centre of Manchester.
Mr Keelan said 100 firefighters from Greater Manchester were tackling seven ongoing blazes on the moorland, with support from Cheshire and Derbyshire.
The fire began on Sunday night, reignited on Monday during the hot weather and then spread throughout Tuesday, fanned by evening winds.
About 150 people were forced to leave their homes on Tuesday, with many staying with family and friends. The majority had returned by Wednesday afternoon.
Jonathan Reynolds, MP for Stalybridge and Hyde, said specialist equipment was being brought by Public Health England to monitor the air quality.
"Residents feel reassured by the actions of the fire service but there is a concern around the air quality and that's something we will monitor," he said.
Mr Reynolds said firefighters were being particularly challenged by "the distance they have to walk and the equipment they are wearing".
"The country needs to give them every bit of support we can," he added.
Mr Keelan said firefighters had carried pumps and hoses up to two miles to tackle remote pockets of fire.
"Firefighters have to physically carry all of that equipment in this heat, and the heat of the fire gear... and that's before they've even started tackling any sort of fire," he said.
"So it's been extremely difficult and they've just got on with it and done a fantastic job, I'm very proud of them."
GMFRS said it was receiving a high volume of calls about smoke across the region and asked for people to only phone in an emergency.
Thanks for your continued support as our crews tackle the #moorlandfires 🙌— Manchester Fire (@manchesterfire) June 27, 2018
Please be aware that due to the smoke being visible across
Greater Manchester @NW_Fire_Control are receiving a huge amount of calls from the public.
Please only call if it’s an emergency 🙏🚨
Mr Keelan added: "There's still a lot of smoke from the fire but air quality levels are being monitored regularly. Air quality is currently at a safe level and therefore residents have been let back into their homes.
"We don't know the cause at this stage and it would be very difficult to ascertain due to the extent of the damage, but we will be looking into that once we have tackled the fire."
Ian Saxon, director of operations and neighbourhoods at Tameside Metropolitan Borough Council, said a maximum of 100 homes had been evacuated at the height of the fire.
"As we're standing here there are people returning and there are people who are deciding they want to go elsewhere," he said.
"We will support them wherever they choose to go."
Personal trainer Kelly McFie, 35, was told to evacuate at 20:00 BST on Tuesday with her seven-year-old son Oscar.
"It looked like a towering inferno when I looked out of the back garden," she said.
"There were huge flames which would occasionally shoot up and you could hear the tree bark crackle."
She said she was "glad to get out" of her home.
"I grabbed our passports and Oscar's birthday certificate and just went," she said. "We've got two cats, Buzz and Sally, and a dog called Bobby.
"Thankfully I managed to grab them although they weren't too happy to be moved."
Grandmother Pauline Lomas, 66, who was evacuated but came home on Wednesday morning to Carrbrook, welcomed the military intervention.
She said: "I'm shocked that they've been deployed as the smoke had subsided now.
"But we can't see how bad it is up on the moors now.
"It's fantastic how much everyone is rallying round to look after us and it shows how seriously they are taking if the Army is getting involved."