Dozens of bonfires lit across Belfast on the Eleventh Night saw dangerous air pollution reach more than double internationally accepted levels.
Pollution in the city began to rise after 10pm on Tuesday and peaked at midnight before dropping and then peaking again at 6am on Wednesday.
Levels of potentially lethal fine particulates soared during monitoring, prompting concerns about the impact on health - particularly for the young, old, vulnerable and sick.
Pollutants known as PM2.5 were detected at four or five times the levels normally seen in Belfast.
“Some problems associated with PM2.5 are lung problems, cardiovascular problems, and recent studies link exposure to fine particles to dementia,” Dr Liz Coleman, from NUI Galway's School of Physics and Centre for Climate and Air Pollution Studies, said.
The levels in the city were found to be more than twice the limit recommended by the World Health Organisation.
The UK’s Department for Environment, Food and Rural Affairs states that there is no safe level for this type of pollution.
It warns that long-term exposure is the biggest issue, but high concentrations over a short period can exacerbate lung and heart conditions, significantly affecting quality of life, and increase deaths and hospital admissions.
Air quality is monitored at seven locations across greater Belfast.
Bonfires lit on the Eleventh Night were still smouldering as Twelfth of July parades got underway on Wednesday, ahead of the clean-up operation.
In some areas, damage was caused to nearby homes with windows cracked or frames melted.
Firefighters were kept busy dealing with incidents and calls peaked at a rate of one every minute.