Special Report

'It is Possible to Fry People': Health Effects of Heat Islands

Hamilton's expanses of asphalt contribute to the heat island effect that puts our citizens at risk during heat waves. Why do we continue to permit them?

By John Neary
Published July 21, 2011

It's unclear whether global warming has anything to do with the current heat wave in Southern Ontario. But it's crystal-clear that urban heat islands are making the problem significantly worse than it needs to be.

Surface parking is not just bad for urban vitality; it also contributes to the heat island effect
Surface parking is not just bad for urban vitality; it also contributes to the heat island effect

The urban heat island effect has been known for two hundred years. Its major causes are succinctly summarized in a 1972 paper in Environmental Research by John F. Clarke:

The brick and concrete that make up a high percentage of the urban surface have a higher heat conductivity and storage capacity than the soil and grass that generally comprise the rural surface: the specific heat per unit volume of concrete is about twice that for dry clay or dry sand, and the thermal conductivity of concrete is about 10 times that of dry clay or dry sand.

During daylight hours, much of the solar radiation striking the rural surface is reflected back into space or is used in evaporative processes or to heat the atmosphere by the processes of conduction and convection from the heated surface. Little of the solar radiation striking the urban surface is used in evaporative processes; because of the high thermal admittance of the urban surface, much of the solar radiation is effectively stored in the urban structure as heat.

After sunset both urban and rural surfaces cool by the radiative process. Because of the large reservoir of heat and counter radiation between buildings, the urban structure cools at a slower rate resulting in an urban temperature excess. Another factor contributing to the urban temperature excess is heat produced within the city by man's activities.

While the above factors contribute to increased ambient temperatures in cities, Clarke also draws attention to lower wind speeds and increased exposure to radiant heat as factors that lead to increased heat stress on human beings.

Tangible Consequences

This discussion would be purely academic if the heat island effect had no tangible consequences for city-dwellers. Yet even in 1972, the relationship between the heat island effect and heat wave mortality was well-documented, as Clarke describes:

The results of several studies suggest that a large percentage of heat-related deaths may be due to climate modification brought about by urbanization: Shattuck and Hilferty (1932, 1933) found that the urban heat-related death rate is higher than the rural heat-related death rate. The heat-related death rate tends to increase markedly with increase in size of the city.

Henschel et al. (1968) found that, of the 246 deaths attributed primarily to heat in the St. Louis metropolitan area during July 1966, 85% occurred within the city limits (which contained about 32% of the population); the death rate due to heat within the city limits was 5 times greater than in the adjacent suburbs. Buechley et al. (1972), in a study of the July 4, 1966, heat wave in the New York metropolitan area, found a high correlation between the spatial distributions of excess deaths and of minimum temperature.

The medical literature even contains reports of people who suffered severe burns from contact with paved surfaces. In 1996, James Berens published three such cases from Phoenix in the Journal of the American Medical Association. Here's the first:

A 22-year-old woman of approximately normal weight was struck by a car while walking across a city street in the early afternoon on June 12, 1968. She was rendered unconscious, and reportedly lay on a hot asphalt pavement for an estimated ten minutes. Her injuries included contusion of the brain with coma, second and third-degree burns present on all four extremities, and second-degree burns on the left side of her face.

Third-degree burns after ten minutes of lying on asphalt. Berens observes that an air temperature of 25 degrees in Phoenix corresponded to an asphalt temperature of 52 degrees - enough to damage skin after 60 seconds of exposure. At 31 degrees air temperature, the resulting asphalt temperature of 62 degrees would damage skin after only five seconds.

As Berens notes:

Most of us have had the experience of accelerating our gait to shorten the time that our bare feet are in contact with hot pavement, and have been witness to expressions such as "You could fry an egg on that pavement." We have not, however, been sufficiently alerted to the occurrence of contact burns in this situation. Indeed, it is possible in hot weather to fry an egg on our streets and highways; furthermore, it is possible to "fry" people.

D. A. Vardy et al reported a similar case from Israel in 1989 in the straightforwardly-named journal Burns:

A 70-year-old woman was admitted to hospital with heat-stroke. She had also sustained deep partial and full skin thickness burns covering 20 per cent of her total body surface area (TBSA) from contact with an asphalt pavement. She had undertaken a 4 h bus ride during a heat-wave. She arrived in the early afternoon and had to walk several hundred metres in the mid-day sun when the air temperature was 43°C (109°F). While walking she became exhausted, stopped to rest and went into a coma. She lay unattended on the asphalt pavement for about half an hour until she was found and rushed to hospital.

How long will we allow our city to remain covered with expanses of asphalt like these? And how long will we continue with transportation planning that demands them?

John Neary lives in Beasley Neighbourhood and practices general internal medicine at St. Joseph's Healthcare Hamilton. He would like Hamilton to develop an urban environment that creates less gainful employment for his profession.


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By mystoneycreek (registered) - website | Posted July 21, 2011 at 12:02:41

This is more dangerous than the winter's frostbite and windchill, because heat of this magnitude can sneak up on you, catch you unawares so easily. I believe I read in The Spec earlier this week that with every degree increase in core body temperature, there's a possible 30 beats-per-minute compensatory increase in heart rate.

Be mindful of your neighbours, especially the elderly. (This is where close-knit communities really pay off.)

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By Charlie Mattina (anonymous) | Posted July 21, 2011 at 12:41:10 in reply to Comment 66716

Agreed, we all must good Neighbours. For those who don't mind getting wet and are in Beasley. There are splash pads in Beasley Park and McLaren Park (John and Cannon.) An Oasis from the asphalt desert.

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By Applause for the cause (anonymous) | Posted July 31, 2011 at 13:27:33 in reply to Comment 66722

insult spam deleted

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By Stay Cool (anonymous) | Posted July 21, 2011 at 14:44:42

Joey Coleman has thoughtfully prepared a map of City pools and splash pads. You can find the map on his website:


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By Applause for the cause (anonymous) | Posted July 31, 2011 at 13:28:24 in reply to Comment 66732

insult spam deleted

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By SpaceMonkey (registered) | Posted July 21, 2011 at 15:48:23

I'm all for increasing the amount of green space and plant life in Hamilton, as well as reducing the number of "needless" parking lots, but...

If the heat islands are dangerous in the summer, couldn't someone turn around and say something like "well then they help to keep the elderly safe in the winter"?

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By Applause for the cause (anonymous) | Posted July 31, 2011 at 13:29:11 in reply to Comment 66742

insult spam deleted

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By UrbanRenaissance (registered) | Posted July 21, 2011 at 16:13:29 in reply to Comment 66742

Snow has a much higher reflectivity than the ground or other common building materials. That combined with the fact the sun is so much further away during winter means that less solar radiation is hitting the ground and being converted to heat, which reduces the UHI effect in winter.

I know this because the company I work for provides consulting services for outdoor thermal comfort (among other things), and we often have to come up with strategies for reducing the Urban Heat Island effect.

Comment edited by UrbanRenaissance on 2011-07-21 16:23:13

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By mystoneycreek (registered) - website | Posted July 21, 2011 at 15:54:25 in reply to Comment 66742

They could.

That wouldn't make them correct. (It might just make them a 'politician'.)

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By John Neary (registered) | Posted July 21, 2011 at 20:48:52 in reply to Comment 66743

I think UrbanRenaissance hit the nail on the head. It's obvious walking around the city on a day like this that it's hotter on asphalt than on grass. But any such difference in the winter is not at all obvious. On a clear winter day without snow cover a parking lot would probably be very slightly warmer than a park but the difference would be much smaller than in the summer.

As a postscript, the one silver lining of parking lots in the summer (in principle at least) is that they probably interfere with wind currents less than buildings do. But in the winter that effect would work against them.

Needless to say, thermal burns from asphalt are a pretty rare occurrence. But the ability of asphalt to cause burns is indicative of how much it contributes to the heat island effect.

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By Applause for the cause (anonymous) | Posted July 31, 2011 at 13:30:06 in reply to Comment 66753

insult spam deleted

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By Undustrial (registered) - website | Posted July 21, 2011 at 23:59:05

It's not just a matter of direct burns. Every extra degree means more risk to the vulnerable. This heatwave has killed over a dozen people so far in the US, but it could be a lot worse. Years back (2003/4?), a heatwave in Europe killed tens of thousands (mostly seniors).

I ventured out today - partly to find some air conditioning (my house wasn't habitable by noon), and partly to witness the carnage. Just before I got home, I took a good look at a tree growing out of the side of a parking lot and imagined, for a second, what today would have been like without the greenspace we do have. In this kind of heat, every tree is an oasis.

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By Applause for the cause (anonymous) | Posted July 31, 2011 at 13:31:03 in reply to Comment 66764

insult spam deleted

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By John Neary (registered) | Posted July 22, 2011 at 07:50:04 in reply to Comment 66764

Hi Undustrial,

I completely agree. I mentioned the burn cases simply as an illustration of how powerfully asphalt contributes to the heat island. Of course heat stroke and heat exhaustion are far, far more important public health concerns.

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By Mr. Meister (anonymous) | Posted July 22, 2011 at 03:56:19

Maybe we need to stop the intensification of our downtown and put in more green spaces. Seems like this whole intense concrete cityscape might not be all that good after all. The upcoming park on Rebecca is a good start. Lord knows we could do without a couple of the parking lots we have downtown.

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By Undustrial (registered) - website | Posted July 22, 2011 at 09:13:30

There are a lot of options here. Toronto's done a lot of good work studying them


In this kind of weather, the need for greenspace goes well beyond grass. Anywhere it isn't being thoroughly watered, lawns are now bright yellow. We need bigger and heartier plants to survive and shade the others. Trees are the most useful, because they not only provide shade and shelter, but also send deep roots underground to bring up water which has seeped out of the reach. You can create deserts by cutting down forests - that's a large part of what happened to the Middle East.

As for our buildings - it wouldn't take much work to safely cover many of them with vines - which adds a lot of summer and winter insulation. Use grapes and hops (which can grow 20cm/week), and you can cool yourself twice...

Comment edited by Undustrial on 2011-07-22 09:16:20

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By MAC Climate (anonymous) | Posted July 26, 2011 at 20:41:44

MAC Climate Change Centre will be hosting a public lecture by DR. TIm Oke on Climate in Cities on October 5th at the MAC Innovation Park. More details shortly at climate.mcmaster.ca

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