Month: August 2018
|This story was produced through a partnership with EarthFix, an environmental journalism collaboration of public media in the Pacific Northwest. It’s part of “Breathing Fire,” a series of research reports and journalism features by Climate Central. The work has received support from the Bill Lane Center for the American West at Stanford University. Read the report.|
Ask anyone who lived in Washington’s Wenatchee Valley in 2012 about the smoke that year, and they’ll remember. The fires were close and the valley’s dry hillsides trapped the wildfire smoke. It was so bad clinics and drug stores ran out of masks. The air was so choked with smoke that summer camps were canceled and children were kept inside.
Anastazia Burnett won’t forget that summer. More than once, asthma attacks drove her to the walk-in clinic for emergency treatment. At the time, she was newly pregnant with her first child.
It was scary, she remembers, “because, when your blood oxygen is low, your baby’s blood oxygen is low, too.”
Climate change is advancing. Snowpack is decreasing, and summers are hotter and drier. A century’s worth of fire suppression is leaving forests overloaded with fuel. All of that is creating the conditions for wildfires to spread quickly and widely and burn huge trees along with the underbrush. Fire seasons are now 105 days longer in the western U.S. than they were in the 1970s. And longer wildfire seasons means more smoke pouring into cities and towns.
So even while air quality has generally been improving across the U.S. since the passage of the Clean Air Act, air quality is getting worse in large swathes of the West during fire season. That is a major threat to public health, because air pollution aggravates conditions like asthma and emphysema, and it can also harm those who were previously healthy.
‘Clearly Linked To Climate Change’
Dan Jaffe, a professor who researches air pollution at the University of Washington in Bothell, has published a paper in a scientific journal this summer that showed that the most polluted days of the year are becoming even more polluted in the Intermountain West.
Dan Jaffe, a professor at the University of Washington, Bothell.
Credit: Katie Campbell, KCTS9/EarthFix
A separate look at air quality data by the non-advocacy news and science organization Climate Central show researchers found a similar trend: In Washington, Oregon and California’s Central Valley, a higher percentage of poor air quality days are occurring during wildfire season.
“These effects are very clearly linked to climate change,” Jaffe says — which means as climate change advances, air quality is likely to get worse.
And poor air quality because of smoke is pretty unavoidable for everyone.
“When the air quality is bad, we don’t get breaks until…the fires get contained or the wind shifts,” says Dr. Craig Kunz, a pulmonologist in Wenatchee. “It’s more of a constant exposure that you can’t really avoid.”
Public health officials advise people to stay inside during smoke waves, but people still have to go outside for groceries, medications and doctor visits. That’s especially true for people like Burnett, with lung conditions like asthma or emphysema that often flare up during smoke events.
Anastazia Burnett with her two children in their home in Waterville, Washington.
Credit: Eilís O’Neill, KUOW/EarthFix
“Your chest starts to feel tight and then it will get tighter and tighter, and you feel really short of breath like you’ve been running a lot,” Burnett says, “and it just doesn’t go away.”
People with lung conditions are more likely to refill their prescriptions, go to the doctor, and be hospitalized during wildfire smoke events. Researchers with the EPA say that, between hospital admissions, emergency room visits, and premature deaths, wildfire smoke exposure costs the U.S. between $11 billion and $20 billion per year.
No One Is Immune
People with pre-existing conditions aren’t the only ones who are affected. Children are also vulnerable because their lungs are developing. Low-income people are at risk as well, in part because they’re less likely to have well-sealed homes and air conditioning.
When pregnant mothers are exposed to wildfire smoke, there’s a “small but significant decline in birth weight,” says Colleen Reid, a geography professor at the University of Colorado in Boulder who researches the public health effects of wildfire smoke.
Reid says scientists haven’t finished teasing out all the other potential health effects of wildfire smoke. They’re most worried about PM2.5, a component of wildfire smoke and other sources of air pollution that is so small it can make its way into human lungs and bloodstream.
Other, more extensively studied sources of air pollution are known to harm cardiovascular health and can increase the risk of obesity and diabetes.
“What we’ve wondered is can these really high air pollution but short time period events such as wildfires impact the cardiovascular system in the same way that chronic long-term exposure to lower levels of air pollution can,” Reid says. Some of the studies that have looked at that question have found an impact — and some have not.
A Fire Strategy To Curb Smoke
Some researchers suggest one way to help the communities hit hardest by waves of smoke for days or weeks at a time year after year would be to change the way forests are managed.
By setting prescribed fires during the shoulder seasons, forest managers can reduce the risk of summer megafires, while burning the forest at times when the wind will carry smoke away from major population centers.
A hazy sky over the Columbia River in Eastern Washington. Fire seasons are now 105 days longer in the western U.S. than they were in the 1970s. That’s one reason smokey skies are more common during wildfire season than in the past.
Credit: Eilís O’Neill, KUOW/EarthFix
And prescribed fires produce less pollution per acre than wildfires, adds Janice Peterson, an expert on wildfire smoke and air quality with the Forest Service. That’s because they’re set when the forest is wetter, so they don’t burn the forest crown or smolder in the forest floor.
Even if prescribed fires would be beneficial in the long-term, though, there are obstacles to setting them.
“The rub,” Peterson explains, is that, “if wildfire smoke pollutes the community for days on end, … nobody is responsible. Nobody gets in trouble.” But, if the Forest Service sets a fire and there’s smoke “for, you know, two, three, four hours or sometimes maybe a couple of days, … we do get in trouble for that.”
Peterson says Washington’s Methow Valley and Bend, Oregon, are particularly resistant to having to deal with any smoke from prescribed fires. Both locations depend on year-round outdoor tourism to fuel their economies, and, in recent years, they’ve attracted many upper-income residents and vacation-home owners.
Resisting Smoke From Prescribed Fire
Both Washington and Oregon have strict smoke management plans that side with communities like the Methow Valley and Bend.
As a result, Peterson says, “a lot of times we don’t burn the amount of acres we would like to because…there’s too much concern about affecting public air quality.”
After the severe wildfires of 2014 and 2015, the Washington Legislature asked the state’s Department of Natural Resources to update the state’s smoke management plan to make it easier to set prescribed fires — and Oregon is also revisiting its smoke rules. That said, it’s complicated.
“Given the strength of our air quality laws, it’s really hard…to say, you know, ‘Yes, we can allow more smoke where humans are going to breathe it,’” Peterson says.
From a public health standpoint, Kunz, the pulmonologist, says short-lived smoke exposure is better than out-of-control wildfires.
“These large fires burn for a long time, and patients are exposed to poor air quality for a prolonged period of time,” he explains. With that prolonged exposure, he says, patients with lung diseases get much worse, and patients who’ve never had lung problems often show up in his office for the first time. But, he says, “I don’t see patients when they’ve had short-term smoke exposure. They’re not coming in and seeking more help or getting into problems or having emergencies.”
Earlier This Year
In the Wenatchee Valley, the smoke came later than usual this year, but it’s here. The early August air is thick and stings your eyes and nose when you walk outside.
Burnett can normally see the Cascades out her back window — but, today, smoke obscures the view.
“You can’t even see Pine Canyon, which is, you know, just like one field across,” she says. “When I opened up the hatchback of my car, you could see ash falling…and that’s how it’s been the past week or so.”
When the smoke gets bad enough, Burnett keeps her kids, who are 5 and 2 now, indoors. She has an air filter going in the living room. She says, sure, they start to get a bit of cabin fever — but summer smoke east of the Cascades is like rain in Western Washington: something you just have to deal with.
Still, she says, “if every year were as bad as 2012, I would definitely consider moving.”
Copyright 2018 Climate Central and EarthFix.
Ivy Albert’s job is conversation. Working as an interpreter, she helps the hearing impaired by translating things like doctor’s appointments and business meetings to and from American sign language. She was also born with asthma.
|This story was produced through a partnership with the Idaho Statesman, a daily newspaper in Boise. It’s part of “Breathing Fire,” a series of research reports and journalism features by Climate Central. The work has received support from the Bill Lane Center for the American West at Stanford University. Read the report.
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Her work requires a lot of talking, something directly impacted by her condition. “When you’re short of breath, it’s really hard to hold a conversation,” she said.
When wildfire season ramps up in Idaho, Albert can feel the toll the smoke takes on her lungs. “It makes me tired,” she said. “I’m working double-time just to breathe.”
Commuting between Boise, where she’s lived for 11 years, and nearby Meridian, she has to travel with the windows down. Her air conditioning doesn’t work; repairing it costs money, money that Albert doesn’t get when she has to give up shifts at work because of her asthma.
Dr. John Jeppson, an allergist and immunologist at the Boise Valley Asthma and Allergy Clinic, has observed people’s struggle with smoke firsthand. 2016 was a bad smoke year in Boise, with winds carrying smoke from the Pioneer and Rough fires in the nearby Sawtooth National Forest into the valley. “We had seven or eight people a day coming into the clinic saying, ‘I’m really struggling,’ ” said Jeppson.
Smoke fills downtown Boise.
Credit: Kyle Green/Idaho Statesman
People that have chronic respiratory issues, people like Albert, are hardest hit. They already have inflammation in their lungs, said Jeppson. “It’s almost like pouring alcohol on a cut.”
“What are we supposed to do,” asked Albert, “do we just live our lives inside?”
The problem is the focus of a report released Wednesday by Climate Central, a research and journalism nonprofit. The group examined air quality indices in four states, including Idaho, between 1999 and 2017 and how they are being impacted by wildfire smoke.
Prevailing winds from the west and northwest carry smoke from wildfires in California and the Cascades towards Idaho. Combined with local fires and the shape of the Treasure Valley that collects and concentrates smoke, Idaho is an epicenter of smoke activity. In 2016, for instance, there were 48 days in which people were exposed to unhealthy levels of fine particulate pollution due to smoke or other causes.
Temperatures are rising as more greenhouse gases from fossil fuels and other industrial sources trap heat. Combined with increasing wildfire activity, the situation is only going to get worse.
Breath of the Dragon
Smoke is actually a complex mixture of materials, a wispy blend of gases, water vapor, and particulate matter.
This matter, the primary health concern for most, is made up of solid and liquid particles, tiny chunks thrown into the air as the fire’s fuel disintegrates. “The person breathing them in may not have a clue what’s going on,” said Loretta Mickley, an atmospheric chemist at Harvard University who is studying the overlap between wildfire smoke and human health. “But, it’s causing these acute, long-term effects that are occurring in the lungs and eating away at overall health.”
The scale of these particles is measured in micrometers — a human hair is about 60 micrometers across. Particulate matter larger than 10 micrometers usually doesn’t make it into your lungs. Mucous membranes act like goalies, trapping large particles in your eyes, nose and throat, causing irritation but preventing the particles from advancing.
The finest particles, smaller than 2.5 micrometers or PM2.5, represent the biggest health risk of all. A few weeks of high exposure to these particles can lead to health consequences for the following year, said Mickley.
They can be inhaled deep into the lungs, said Dr. Patrick Weis at St. Luke’s Idaho Pulmonary Associates Clinic. “Whatever compounds are there, they get ingested by the cells which sets off an inflammation process.” Those chemical compounds get into tissues and the bloodstream, where they can cause a sundry of health issues.
For instance, chronic obstructive pulmonary disease is on the rise. “It’s in the top five conditions that people die from,” said Weis. Generally thought of as a smoker’s disease, more people who have never smoked a day in their lives are developing the ailment. “As people live longer, they are being exposed to worse air.”
It’s not just lung issues that concern Weis. Increased levels of smoke have been linked to cardiovascular events such as heart attack and stroke. Pregnant women and people with diabetes are also at risk. With decreased air quality “you’re under stress, you have higher blood pressure.” You don’t even have to be sick to be at risk, said Weis.
“When I drive to work and I can’t see the Foothills,” said Weis, “I just know it’s going to be a bad day.”
Studies show that PM2.5 levels across the Pacific Northwest are intimately connected, so the smoke that’s irritating your lungs could have traveled hundreds of miles, maybe thousands. Smoke from a current wave of wildfires in Siberia is being carried across the Pacific Ocean by the jet stream, affecting the Pacific Northwest and Canada.
Along its journey, chemicals in the smoke interact with the atmosphere and sunlight, creating ozone-rich “aged” smoke. This pollutant is also a respiratory irritant, compounding the problem from PM2.5.
Where There’s Fire, There’s Smoke
“Fire is certainly on the rise,” said John Abatzoglou, an associate professor of geography at the University of Idaho who advised on the Climate Central report. Compared to an average year from the 1970s, Idaho is now experiencing more than 10 times the number of 1,000-acre-plus fires. It’s not just Idaho, it’s the entire Pacific Northwest, with Oregon experiencing nearly seven times and Washington nearly five times as many.
Fire season in the West has typically been between Memorial and Labor days, but this is changing. The average fire season is now 105 days longer than it was in the 1970s. More fire means more smoke.
Overall air quality across the nation has improved due to environmental regulation and improved technology. However, the air quality in the West during fire season is getting worse. In Idaho, more of the days when the state’s average air quality is deemed unhealthy due to PM2.5 are now happening during peak fire season.
Smoke was Ed Delgado’s first exposure to wildfire. He grew up in central Texas, which became inundated with smoke from the Yellowstone wildfires of 1988. Delgado is a meteorologist and was working at a forecast office in Fort Worth at the time. “It was smoky, it was hazy, you could smell it,” he said.
Delgado is now the national program manager for Predictive Services, the top meteorologist at the National Interagency Fire Center in Boise. Over his 32-year career, he’s seen drastic changes in the way fire and smoke play out in the West. He’s seen larger fires, and fires that burn longer and throw up more smoke for extended periods of time. This smoke is being carried across the nation and has begun to call attention to the issue.
“Out West, people are used to smoke in the summertime,” said Delgado, “but when you smoke out Chicago or Washington, people take notice.”
A wildfire is much like a campfire. To successfully build one, you need fuel, you need that fuel to be dry, and you need a way to light it.
One fuel playing a major role in the huge range fires of West is cheat grass. This invasive species covers vast swaths of North America, more than 150,000 square miles in the U.S. alone, almost as much land as the entire state of California. It crowds out native species and dries out quickly as fire season begins, becoming prime fuel. It also burns more efficiently than native plants, said Abatzoglou, consuming almost double the area than native plants under the same conditions. After the fire, it can recover faster than native species, continuing the cycle.
Fire crews battled a grass fire south of Boise this month fueled by invasive cheatgrass.
Credit: Shawn Raecke/Idaho Statesman
Increasing temperatures and changing rain and snowfall are making a bad situation worse. Climate models predict average temperatures in the Pacific Northwest will climb by between 4.7 and 6.5 degrees by the time people born today are in their 80s. In addition, the regional climate is getting drier. Abatzoglou estimates that about half of this increased aridity seen since 1979 is due to human-caused climate change.
These changes in climate harm forests as well. Increased aridity prevents trees from efficiently pulling in moisture. “Rather than try,” said Delgado, “they go dormant as a way to protect themselves.” These dry, dormant trees are just waiting to burn.
It’s estimated that humans are responsible for starting 84 percent of all wildfires. Natural climatic variation plays a role as well, amplified by human-induced climate change. Another culprit is build-up of fuels thanks to fire suppression strategies over the years. “We have a debt to pay,” said Abatzoglou of this added fuel load.
The whole situation is a self-fueling feedback loop. Fires release greenhouse gases like carbon dioxide into the atmosphere, amplifying climate change. Losing forest and grassland to fire means less biomass to remove those greenhouse gases. “Fire is not going away, and it’s only going to increase,” said Abatzoglou.
Smoke in the Valley
The Idaho Shakespeare Festival’s outdoor amphitheater is nestled along the Boise River in a natural area full of native plants and animals. Actors on the stage must project their voice to more than 700 people, a feat that considerable lung capacity and control. “These people are in incredible physical shape,” said Hannah Read Newbill, the festival’s marketing director.
Late summer of 2017 found Boise inundated with smoke from distant fires, trapped in the Treasure Valley by inversions. “You could taste it,” said Newbill.
During the company’s run of “The Hound of the Baskervilles,” the actors started to feel it. The play was “a real physical performance” according to Newbill, requiring even more from the actors than normal. Concerns about the smoke spurred talk of moving the play inside, but the situation improved.
Inversions are a common occurrence, concentrating pollutants and their health effects. Jacob Wolf, a meteorologist at the Idaho Department of Environmental Quality, was hired to monitor inversions and update air quality reports for the state. As the sun sets, said Wolf, the smoke from surrounding fires drains down into the lower elevations. “It’s going to behave a lot like water,” he said.
The dryness of the air in Idaho and other parts of the Pacific Northwest worsen inversions; this air transfers less heat, increasing the chances of smoke concentrating in one area.
Taking Back the Air
With distant sources, unfavorable topography and global climate change, what is an Idahoan to do?
One technique to reduce smoke levels, of course, is to reduce the amount of fire. Prescribed burning is one important step, said Mark Boyle. As the smoke management program coordinator for the Idaho Department of Environmental Quality, it’s his job think about ways to reduce smoke and issue permits to people who want to burn.
Smoke from the Pioneer Fire in 2016 polluted southern Idaho.
Credit: Pacific Southwest Region 5/Flickr
Prescribed burning removes the fuel, turning one potentially huge fire into many small burns, and possibly reduces the overall smoke load. Burners must still be aware of weather conditions, especially in an inversion-prone area such as the Treasure Valley. People want to minimize the effect of smoke on their neighbors, said Boyle, “but they don’t know how weather can affect overall air quality.”
Jeppson and Weis agree that when the air quality is bad, people need to stay inside as much as possible. “People have to listen to their bodies,” said Weis. Making sure that furnace filters are high quality goes a long way, but without specialized systems, most home furnaces can’t filter out PM2.5 or ozone effectively. There are also mental health issues, like depression, tied to staying indoors for prolonged periods of time.
Using in-home HEPA filters is another good choice, but people with respiratory problems should stay away from ionic purifiers. The mechanism these purifiers used to clean the air generates ozone and can compound breathing problems of sensitive individuals.
When we deal with torrential rain or snow, said Abatzoglou, we modify our behavior. “Maybe we need to live in a different way when there are fires burning.”
Ivy Albert thinks that awareness is the key: not only of the respiratory issues people like her suffer, but how to responsibly live in wildfire country. People should understand how to avoid causing fires, she said.
“If it keeps getting like this… I don’t know,” said Albert. “I might just have to leave Idaho.”
Copyright 2018 Climate Central and Idaho Statesman
Research Report by Climate Central
This report is part of “Breathing Fire,” a series of research reports and journalism features by Climate Central.Download report PDF
In more than half of the s…Read More
|Read Climate Central’s research report, “U.S. Faces a Rise in Mosquito ‘Disease Danger Days.’”
Download story PDF here
By John Upton (Climate Central) and Kyle Jones (El Paso Times)
Elisa Sierra had just given birth to twins when she became infected with West Nile virus.
The infection left Sierra, who lives on the West Side of El Paso, Texas, with meningitis and damaged her brain.
Sierra is a clinical social worker who used to provide therapy. She’s in a similar line of work following her 2015 illness, but she doesn’t provide counseling any more because she’s no longer confident she can think quickly enough.
“You have to be able to think on your feet,” she said. “I have trouble remembering simple words.”
West Nile survivor Elisa Sierra at her home in El Paso, Texas.
Credit: Ruben R. Ramirez/El Paso Times
City data shows Sierra is one of about 100 people in El Paso who have been affected by a severe form of West Nile since 2000, which was around the time it was discovered in the U.S.
As El Paso and other governments work to reduce infections, a new analysis by Climate Central shows rising temperatures have increased the number of days each year when mosquito-borne diseases are likely to be spread across the United States.
Some of the greatest increases in risk occurred in El Paso.
Climate Central analysts examined the number of days each year that temperatures were optimal for the transmission of mosquito-borne diseases, which is between 61°F and 93°F. There were more than 240 such days in El Paso last year.
Of the 244 cities analyzed, most were found to have experienced an increase in the average number of mosquito disease danger days each year since 1970. El Paso’s increase of 33 days was a larger rise than all but four other cities — Reno, Nev., Las Cruces, N.M. and San Francisco and Santa Maria, Calif.
In El Paso, West Nile is the mosquito-borne disease of main concern. In other cities around the world, Zika, dengue, chikungunya, Yellow Fever and Eastern Equine Encephalitis viruses are also spreading. Each of these diseases can cause crippling health problems, and each of them depends on specific environmental conditions to thrive.
“Transmission of mosquito-borne diseases depends on a bunch of different biological processes,” said Marta Shocket, a Stanford University biologist investigating the role of temperature on the spread of diseases by mosquitoes.
“It depends on the how long the mosquitoes are living, how many mosquitoes there are, how much they’re biting,” Shocket said. “All of those processes depend on temperature.”
Credit: Dunpharlain/Wikimedia Commons
Temperatures are rising because of pollution from cars, power plants and logging. As the Trump Administration weakens climate protections, El Paso is among more than 1,000 cities pushing through a national association of mayors for aggressive steps to address climate change.
While risks of contracting diseases from mosquitoes have risen with temperatures, it’s difficult to measure the impacts.
Other factors also influence transmission rates, such as efforts to reduce mosquito populations, keep them out of homes and eliminate standing water. And diseases continue to emerge and spread into new areas.
An El Paso man, who asked that his name be withheld when discussing his medical history, was among 14 people in El Paso diagnosed last year with the serious neuroinvasive disease that the West Nile virus can cause.
The man said he didn’t realize he’d been bitten by a mosquito until the West Nile virus reached his brain, nearly killing him.
“I didn’t have any welts,” he said.
Following visits to an emergency care clinic as he suffered a neurological breakdown, he was hospitalized and diagnosed with the most serious form of disease the virus can cause, which occurs when the virus invades the nervous system.
He had to learn to walk again and is sensitive to light. It hurts his eyes, and sometimes exposure makes him sneeze uncontrollably. He suffers from aches and has become more vulnerable to other health problems.
“I feel like I’m forever inside a cotton ball, everything’s just fuzzy,” he said. “I wound up with a bout of Bell’s Palsy. I lost sensation in half my face, and I wasn’t able to speak anymore. I was slurring my words.”
City officials spray pesticides to control mosquito populations and urge residents to prevent the pests from breeding by checking for standing water. They also suggest using bug spray and wearing long-sleeved shirts and long pants to avoid being bitten, particularly around dusk and dawn.
Vector Control code compliance officer Carlos Sarmiento carries a mosquito trap out of a ponding area along Tierra Dorada Drive across from Chester E. Jordan Elementary School in East El Paso. Sarmiento counted seven mosquitoes trapped inside.
Credit: Rudy Gutierrez/El Paso Times
Doug Watts, a researcher at the University of Texas, El Paso, has been working with a pediatrician to test samples from new mothers for West Nile antibodies. The samples suggest that just 4 percent of El Paso residents have been infected during their lifetimes.
Perhaps counterintuitively, the limited exposure of El Paso residents so far to West Nile virus can make them more vulnerable to the disease, Watts said.
“The prevalence of this virus in the community appears to be very low,” Watts said. “Because it’s very low, you don’t have what we call a high prevalence of herd immunity. There’s a lot of people that are still susceptible.”
Three years after her infection, Sierra is angry that her condition wasn’t diagnosed by medical staff more quickly. She’s also frustrated that it’s hard to find specialists who can help her.
“Before I got the disease, I figured it only affects babies and old people,” Sierra said. “It’s important for people to understand that it doesn’t matter your age. You need to protect yourself.”
Sierra is slowly recovering from the disease, but she expects it will affect her for the rest of her life.
“I had to learn how to write again,” Sierra said. “I lost brain matter.”
This story was produced through a partnership with the El Paso Times.
Research Report by Climate Central
|Download report PDF|
Among the many consequences of human-caused climate change is a change in the pattern, incidence and location of some diseases spread by biting mosquitoes, ticks and flies. These diseases pose a significant public health challenge globally, including in the United States.
The number of mosquito “disease danger days” is increasing across much of the U.S. as temperatures rise, representing a greater risk for transmission of mosquito-borne diseases. And even though mosquitoes are often just an itch-inducing nuisance, the consequences can be deadly.
Mosquitoes are major carriers of these diseases, with a variety found throughout the U.S. While Culex mosquitoes are found across the United States, two species, Aedes aegypti and Aedes albopictus, which carry some dangerous diseases and thus are the subjects of many studies, have a more limited range in the U.S. These two species tend to be found in the South and Southeast, though the former’s range extends into California and the latter’s range extends northeast towards New York and New England and has the potential to exist in the Midwest. Although other species of Aedes mosquitoes exist, in the remainder of this report, Aedes will refer to only Aedes aegypti and Aedes albopictus.
To examine the role temperature is playing in disease transmission from mosquitoes, Climate Central analyzed the number of days each year in the spring, summer, and fall with an average temperature between 61 degrees and 93 degrees Fahrenheit. This is the range for transmission of diseases spread by mosquitoes of the Aedes or Culex type. Of the 244 cities analyzed, 94 percent are seeing an increase in the number of days, indicating a heightened risk for disease transmission, or “disease danger days.”
Diseases spread by mosquitoes are known as “vector-borne diseases,” as they are spread by an infected agent that serves as the “vector.” There are nine diseases carried by Aedes and Culex mosquitoes reportable to state and territorial health departments in the U.S. — seven of these nine diseases have already been transmitted in the U.S. Both types of mosquitoes transmit West Nile virus, with cases in all 48 continental states since its introduction to the U.S. in 1999. Aedes mosquitoes also carry other dangerous diseases such as dengue, Zika, chikungunya, and Yellow Fever, while Culex mosquitoes also transmit St. Louis Encephalitis and Eastern Equine Encephalitis viruses.
These diseases are more often found in U.S. territories (like Puerto Rico) than in the continental U.S., but there has been limited transmission of Zika, dengue and chikungunya viruses in Texas, Florida and Hawaii. And while these diseases may not always be life-threatening or even present symptoms, there can be serious consequences without proper medical care.
In addition to needing the proper climatological factors for the mosquito to survive and transmit disease, there needs to be the establishment of the disease in the first place — having the proper climatic conditions, a critical density of mosquitoes, and the conditions for the sustained cycle of disease transmission itself. And, in order to transmit disease, a mosquito must bite twice — once to acquire the disease themselves, and a second time to pass it on. The largest number of these twice-biting mosquitoes were produced at 75 degrees Fahrenheit.
Disease Danger Days
Regardless of mosquito type, there’s an elevated risk of disease transmission when temperatures are between 61 degrees and 93 degrees Fahrenheit. Climate Central analyzed the number of days with temperatures in the risk zone during spring, summer and fall. Of the 244 cities, 94 percent (229 cities) have an increasing number of disease danger days.
Trend in mosquito disease danger days in these U.S. cities
Three California cities (where both types of mosquitoes have already been found), rank numbers two, three, and nine on the list of biggest increases in disease danger days. Other cities in the top 10 list include southern and southwestern cities like Las Cruces, New Mexico, El Paso, Texas and Tucson, Arizona, where temperatures are suitable for mosquito survival and some disease transmission already occurs. Our analysis suggests that since the 1970s, the risk of disease transmission in these cities has been increasing due to rising temperatures.
A few northern cities also make this list of greatest increases in disease danger days, including Helena, Montana, and Erie, Pennsylvania. Other northern cities that didn’t make the top 10 list but have a large increase in the number of disease danger days are located in states such as Idaho, Vermont, Washington, New York and Michigan. Some cities in these states are notching more than 20 additional disease danger days, demonstrating the wide-reaching threat climate change holds.
Of the possible 275 days analyzed for this report, Honolulu and San Juan fall in the disease transmission zone for the entire stretch. In fact, dengue transmission is possible year-round in San Juan.
Importantly, climate change may also actually make some locations too hot for mosquito survival and disease transmission. For example, there are fewer disease danger days in Phoenix since 1970. This is likely because the number of days above 95 degrees Fahrenheit in Phoenix is also going up, and 95 degrees is beyond the range of disease transmission.
Out of the 244 cities in the analysis, only 12 (5 percent) are seeing a decrease in the number of disease danger days during this time period.
Honing in on transmission, rates for mosquito transmitted diseases peak between 79 degrees and 84 degrees Fahrenheit. Of the cities analyzed in this report, Honolulu has the greatest increase of days in the peak range, with an additional 36 days each year, on average, during spring, summer and fall. Many of the other cities recording additional peak transmission zone days are in the Southeast, such as Raleigh, North Carolina and Atlanta, Georgia (26 and 25 additional days, respectively).
Though 88 cities have a decline in days in this peak range between 1970 and 2017, many of these are still seeing an increase in the number of days in the total transmission range. Also, a number of cities with fewer peak days may also simply be getting hotter than this range, as with Phoenix.
Our analysis did not consider winter temperatures, and in some of the cities analyzed, diseases can still be transmitted during that time. As climate change increases temperatures during winter months, transmission could become possible year-round in some places across the continental U.S., beginning with South Florida.
The Effect of Changing Temperatures
Temperature plays a major role in the viability of mosquitoes’ range and survival, and can affect mosquitoes at every stage of their life cycle. Because of this, rising temperatures due to climate change are changing mosquito habits and disease spread.
For Aedes mosquitoes, development and survival is limited to temperatures above 50 degrees Fahrenheit and below 102 degrees for Aedes aegypti, and above 59 degrees but below 95 degrees for Aedes albopictus. For these two related species, peak rates for various parts of their life cycle generally occur between 73 degrees and 93 degrees.
Culex mosquitoes thrive in temperatures between 50 degrees and 95 degrees.. Though adults survive for the longest time at temperatures between 60 degrees and 68 degrees, studies have found that they develop fastest at temperatures between 82 degrees and 89 degrees.
Hotter temperatures also generally decrease the time it takes for a virus to be transmittable from the mosquito to humans. Rising temperatures shorten how long it takes for the virus to develop inside the insect, known as the Extrinsic Incubation Period (EIP), which increases the number of mosquitoes that survive long enough to become infectious. Each virus has a unique EIP which is optimized at a different temperature. For example, the EIP of dengue virus is shortest at 95 degrees. Increasing temperatures have been shown to increase West Nile virus infection, dissemination and transmission rates up to at least 89 degrees. Zika virus’s optimal EIP and range go even hotter — peaking at 97 degrees.. Mosquitoes carrying the Zika virus would still become infectious at 108 degrees, except mosquitoes themselves cannot survive in that heat. Still, by changing life cycle rates and EIP, temperatures alter the transmission rates of these dangerous diseases.
Climate Change, Mosquitoes and Health
Overall, the number of mosquito disease danger days is increasing across much of the U.S., representing a greater risk for transmission of mosquito-borne diseases. Dengue, Zika, chikungunya and West Nile viruses all represent significant health threats globally, and while they are currently limited in the U.S., could become more major problems if climate change is not abated.
A Zika virus epidemic began in May 2015, spreading through parts of South America, Central America and the Caribbean, including Puerto Rico. Though the continental U.S. managed to escape the worst of the epidemic, nearly 6,000 cases were still reported. This epidemic brought the major health complications that mosquito-borne infections can cause to the forefront. With the expected expansion of Aedes mosquitoes’ ranges in North America, especially in eastern North America, it is possible that transmission of Zika virus could be seen in those places in the future.
The land area of the U.S. most suitable for Aedes albopictus mosquitoes is projected to increase from 5 percent to about 50 percent by 2100, putting 60 percent of the northeastern U.S.’ population at risk for the diseases carried by this mosquito, including West Nile virus, dengue and Zika. Overall, West Nile virus transmission in the U.S. is also projected to increase due to warmer temperatures and less precipitation from climate change.
Social and health care infrastructure could assist in limiting disease spread. For example, it is likely because of social factors such as housing infrastructure and public health services that we do not see the same level of dengue transmission in South Florida that occurs in San Juan, considering that the climatic conditions of the two locations are similar.
Methodology: Temperature trends were calculated using average daily temperature data from the Applied Climate Information System (rcc-acis.org). Years with more than 30 days of missing data were excluded from the analysis. The temperature range of disease transmission (61-93ºF) as well as the range for peak transmission (79-84ºF) were chosen based on the findings in Mordecai et al. 2017.
Climate Central’s Jennifer Brady contributed data analysis for this report.
We gratefully acknowledge Marta Shocket, postdoctoral scholar in Erin Mordecai’s lab at Stanford University, for reviewing the analysis and providing feedback on an earlier version of this report.