Heat-related deaths in hottest major US city have almost doubled compared with the same period last year, after Phoenix experienced its hottest ever June on record.

The number of possible heat deaths reported by the Maricopa county medical examiner was 175 as of 29 June – a staggering 84% increase over the same period last year.

That current total comprises 162 suspected deaths under investigation and 13 confirmed heat deaths.

The death toll from extreme heat has been rising every year in Maricopa county, where Phoenix, the Arizona state capital and America’s fifth largest city, is located.

Deaths are concentrated in Phoenix despite city and statewide efforts to tackle the rise in heat mortality and morbidity.

Last year, Phoenix suffered a month of consecutive days over 110F (43C) and a record 645 heat deaths in the county – a 700% rise over the past decade. But the unprecedented July heatwave followed a cooler-than-normal June, which could partly explain the surge in early season heat-related deaths recorded so far this year.

The average temperature – a measure which takes into account the daily highs and lows – made June 2024 the hottest ever recorded in the city. This was driven in large part by multiple record warm lows or night-time temperatures towards the end of the month, when moisture from a tropical system sat over the region, trapping the daytime heat and limiting cooling.

The night-time low on 27 June was 95F – which broke the record for the warmest overnight June low, which was 93F and set in 1990. The hotter nights are particularly alarming for public health experts because heat is cumulative, and the body cannot start to properly recover until the temperature drops below 80F.

Dangerously hot temperatures are forecast for Maricopa county and the entire south-west United States, with a heat dome – a strong area of high pressure and warm air – expected to linger until the middle of next week, according to the National Weather Service (NWS) in a post on X.

With no rain forecast, temperatures could top 115F in Phoenix over the next few days, with records likely to be broken in parts of California and Las Vegas, according to Gabriel Lojero, a meteorologist at the NWS Phoenix.

“The heat is going to be very severe across the majority of the south-west with an excessive heat warning in place for most of the region. The heatwave will be especially severe over the July 4 weekend when more people are doing outdoor activities,” said Lojero.

Heat deaths are rising across the US – and globally – with official counts likely to be a significant underestimate as many medical examiners have yet to develop processes that match Maricopa county’s.

So far this year, heat has also killed at least two people in Idaho and at least five in Kansas City, according to reports from local health officials.

Investigating suspected heat deaths takes time.

Of the 13 confirmed heat deaths in Maricopa county so far this year, 46% of the people were Black, Latino or Indigenous American; 69% were men and 31% were women. At least two deaths took place indoors. In one case, the deceased’s air conditioning was turned off, in the other case it was not functioning, according to preliminary investigations.

Paramedics have responded to 424 heat-related emergency calls so far this year, according to the Phoenix fire department.

It’s not clear what impact the city’s decision to evict a large homeless encampment from downtown – where many services are located – will have on heat deaths, as 45% of last year’s fatalities involved unsheltered people. So far this year, 31% of the confirmed deaths involved unhoused people. It’s unclear whether some unhoused people have left due to Phoenix’s increasingly draconian crackdown on homelessness.

In May, advocates condemned Phoenix for effectively criminalizing homelessness after the Democrat-run city council voted unanimously to ban people from sleeping, cooking or camping at city buildings, parks and parking lots, and also within 500ft of schools, daycare centers, shelters and parks.

The tent ban can now be enforced after the US supreme court last week ruled that it was not unconstitutional for cities to arrest and fine people for sleeping in public spaces, even when no shelter space exists. Phoenix “will continue to offer shelter and services when possible, and will ask for compliance before resorting to an arrest”, according to the city attorney Julie Kriegh.

The region is accustomed to a hot desert climate, but daytime and night temperatures are rising due to global heating. The deadly consequences have been exacerbated by decades of unchecked urban development that created a sprawling heat island, which the city is trying to tackle with a variety of programs.