A petrochemical company has apologised for unplanned flaring at its Fife plant.
The flaring started at 22:00 on Monday at the ExxonMobil Chemical Ltd facility at Mossmorran.
Residents said "light pollution turned night into day" and "extensive noise" meant they were unable to sleep.
It follows three days of planned flaring by Shell, which shares the site and caused no reports of complaints from residents.
James Glen, chairman of the Mossmorran Action Group, told the BBC Scotland news website that the flaring was still continuing.
He said: "Once again the night sky in central Fife has been lit up with apocalyptic flaring as ExxonMobil experiences yet another unplanned emergency.
"A massive plume of black smoke testified to the release of a large quantity of unburned hydrocarbons and other toxic pollutants.
"Families in neighbouring communities were again forced to suffer extensive noise intrusion, light pollution that turned night into day and consequent sleeplessness and anxiety."
50 miles or 80km from Mossmorran in Scotland. Seems they like to light up the sky at 22:30. and make an absolute racket too with flaring.— Bob Gallagher #FBSI #FBPE (@bobgallagher18) August 12, 2019
This is actually my view from the edge of the Highlands in Bankfoot pic.twitter.com/c1BePu7MWH
Mr Glen added: "Residents were left frustrated when they couldn't report the unplanned emergency to SEPA as their pollution helpline proved unable to cope with the volume of calls.
"Individuals have submitted reports to our group of disturbed sleep, nausea, stress, anxiety, headaches, sore throats and burning eyes during this extreme flaring event.
"In 2001 the then shadow health minister, Nicola Sturgeon promised residents a public probe into Mossmorran, 18 years later residents are still waiting.
"Public calls for an independent social impact study has fallen on deaf ears, as the first minister, and health minister Roseanna Cunningham ignore the plight of residents."
Stuart Neill, external affairs manager for ExxonMobil, said: "We can confirm that we have experienced mechanical failure across two of our three boilers.
"With the loss of this steam generation, we cannot continue to operate our normal processes.
"As such, we are progressing with the unit shut down to allow us to evaluate the root cause and execute necessary repairs.
"Flaring will be required while our team take the steps required to safely shut down operations.
"We will always work to minimise the timelines, and will update you as these are confirmed.
"We, again, apologise if flaring is causing any concern but reiterate that the process is safe and poses no risk to communities."
A Scottish government spokesperson said: "We are clear that prolonged, unplanned flaring is unacceptable.
"The Scottish Environment Protection Agency is currently responding to the latest flaring incident and carrying out a formal investigation into prolonged unplanned flaring earlier this year.
"We are awaiting the conclusion of that investigation before deciding on the most appropriate course of action."
A Sepa spokeswoman said: "We have made clear that ExxonMobil must take steps to minimise the impact of flaring that is currently ongoing and which is expected to continue across the evening.
"Specialist teams are monitoring air quality and, as with previous incidents, will monitor noise across evenings in line with evidential monitoring standards.
"Initial air quality monitoring continues to be in line with previous monitoring and shows no cause for concern.
"The focus of a live regulatory review, officers are currently gathering data to an evidential standard.
"A current 'Best Available Techniques' assessment and operating permit variations will drive action and flaring mitigation investment at the facility."