The amount of dangerous air pollution over Belfast during bonfire night was more than double internationally accepted levels.
Over a period of more than six hours, monitors recorded how the amount of potentially lethal fine particulates soared - raising concerns about health effects on the young, old, vulnerable and sick.
Data from across the city, where dozens of huge bonfires were lit last night, revealed pollutants known as PM2.5 - about one fortieth the width of a human hair - were four or five times the levels normally seen in the city.
The pollution began to rise after 10pm and peaked at midnight before dropping and peaking again at 6am this morning.
Dr Liz Coleman, from NUI Galway's School of Physics and Centre for Climate and Air Pollution Studies, said PM2.5 was recorded at levels more than twice the limit recommended by the World Health Organisation (WHO).
"Some problems associated with PM2.5 are lung problems, cardiovascular problems and recent studies link exposure to fine particles to dementia," she said.
"Obviously, health effects are more severe in vulnerable groups, the very old and very young and those with pre-existing conditions."
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 in greater Belfast.
Firefighters dealt with 40 bonfire-related incidents - up 21% on last year - on a night when they received 213 emergency calls in total and mobilised to 133 incidents overall - a 49% hike on 2016.
The Northern Ireland Fire and Rescue Service (NIFRS) described the night as "exceptionally busy", with crews dealing with 95 operational incidents between 10pm and 1am.
During the most intense period, the NIFRS's regional control centre handled an emergency 999 call every minute.
The NIFRS said its two most significant bonfire incidents were in the greater Belfast area.
The service also reported one attack on a fire appliance during the night - though no-one was injured.