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There's a lot you can do if you're an allergy sufferer, but your city needs to play its part as well.

This year's ranking of Spring Allergy Capitals is in, and it's bad news yet again for allergy sufferers in the Southern half of the United States. Jackson, Mississippi, and Louisville, Kentucky, top the 2015 list of the "most challenging places to live with allergies," published annually by the Asthma and Allergy Foundation of America. Oklahoma City, Memphis, and Knoxville round out the top five.

What makes these cities especially allergenic? Each has its own unique cocktail of airborne allergens, of course. Tree pollen is a major culprit in seasonal allergies, and certain types of trees, such as mulberry and olive, are more allergenic than others. There are also factors that tend to exacerbate symptoms across the board. Air pollution is believed to intensify allergic responses, and geography can compound the effect; Louisville, for instance, tends to trap allergens because of its location in the Ohio River Valley. Climate change also plays a role: increases in temperature, carbon dioxide, and precipitation stimulate the growth of some allergenic species and lengthen pollination periods.

The skyline of downtown Louisville, Kentucky. Not pictured: Allergens. (Photo courtesy Flickr/mshiffphotography)

Meanwhile, the widespread practice of "botanical sexism," or the predominant use of male plants, can make entire swathes of urban landscape inhospitable to people with seasonal allergies. Many communities favor male trees and shrubs because, unlike females, they don't produce fruit or seeds, which can clutter sidewalks or attract pests. But male plants are the ones that produce pollen, and when they're planted in high concentrations, they can cause significant trouble for allergy sufferers.

It's likely that some combination of these types of environmental factors—along with genetic predisposition—is behind your sneezing, congestion, and eye irritation this time of year. But although the causes are diffuse, there are concrete steps that cities and sufferers can take to keep seasonal allergies in check.

What Cities Can Do

More trees, more biodiversity. Trees act as natural air filters, removing pollutants, such as ozone and particulate matter, that can trigger allergic reactions. A 2014 study conducted by the USDA Forest Service suggested that this cleaning power "can produce substantial health benefits and monetary values across the nation, with most of the health values derived from urban trees."

But things get complicated once you factor in the pollen that trees produce. A 2013 Columbia University study, for instance, demonstrated a positive correlation between tree canopy cover and allergic sensitization to tree pollen in a New York City birth cohort. In another study, conducted in Cincinnati, infants exposed to tree pollen in their first year of life were five times more likely to develop hay fever before age 3.

So are trees good or bad for seasonal allergies? The answer likely depends on what kinds of trees you plant, and where. The Columbia study wasn't able to determine how individual tree species, or the spacial distribution of tree pollen, might affect allergic responses. But recent research supports the idea that greater biodiversity can improve human health as a whole. In 2014, a team at the University of Helsinki found a strong association between environmental biodiversity and atopy, or the genetic tendency to develop allergic diseases. The science is still far from definitive, but it does suggest that we need more (and more different) trees, not fewer trees.

More low-allergy plants, fewer high-allergy plants. Cities could focus on planting street trees with lower allergy potentials, such as the tulip poplar, dawn redwood, or hawthorn. Best bets include plants that are insect- rather than wind-pollinated, have short bloom periods, and emit "sticky" pollen, which doesn't travel as easily through the air.

This pollen monster, also known as a mulberry tree, is not your friend. (Photo courtesy Flickr/James Petts)

Communities should also avoid planting the worst pollen offenders. Several cities, including Las Vegas, Albuquerque, and Tucson, have pollen control ordinances that prohibit the planting or sale of highly allergenic plants, such as mulberry and olive trees. But these regulations may have limited impact if they're unenforced or if pre-existing trees are grandfathered in. The key is to plant strategically and to enhance biodiversity overall.

Gender equality. Horticulturist Thomas Ogren has studied trees in the U.S., Canada, Israel, New Zealand, and other parts around the globe, and in city after city he's seen the same phenomenon: a preponderance of male, pollen-producing clones. "In all of the wild lands, there was [gender] balance," he says. "In all of the cities, there was no balance."

Ogren, who created the plant allergy scale used by the USDA, contends that planting more female trees and shrubs can dramatically reduce a city's allergy impact. Female plants are important not just because they don't produce pollen; they also trap pollen and remove it from the air. If cities are concerned about street litter, they can use sterile female trees, which won't produce fruit. It's also not necessary to chop down male trees to achieve gender balance. Ogren says there are ways to graft or prune male trees into female trees, and urban gardeners can be trained to perform such "sex changes."

"Almost every city I’ve ever seen could have used more trees," says Ogren. "I just would like to see them be more intelligent about [planting them]."

Tree spacing. Avoid planting many trees or shrubs of the same species at close quarters. These highly concentrated "living screens" are capable of producing massive amounts of pollen that can't easily be dispersed by air currents. To combat this effect, researchers at the University of Granada recommend that planters abide by certain minimum distances between trees, as well as between trees and buildings.

Grass and weeds grow in front of a vacant home in Detroit, Tuesday, May 27, 2014. (AP Photo/Carlos Osorio)

Smarter maintenance. Vacant lots, derelict buildings, and other marginal urban areas can become major pollen emitters if invasive or highly allergenic species such as ragweed take over. The solution, according to the same Granada researchers, isn't to eliminate spontaneously growing flora, but to manage sites holistically, "encouraging colonization by diverse naturally occurring species and avoiding the establishment of monospecific populations of allergenic species."

A recent study of vacant lots in Detroit seems to have borne this out. Ragweed, one of the nastiest hay fever triggers, had come to dominate much of the city's abandoned real estate, and it was only sporadically mowed over. Researchers at the University of Michigan found that the city's spotty mowing regimen actually encouraged the growth of ragweed, and that no mowing was preferable to occasional mowing since it allowed other native plants to outcompete the ragweed. The implication for cities was clear: either mow regularly, or don't mow at all.

Community data. In Louisville, citizens are using their inhalers to help guide air quality policy. The city's groundbreaking Asthmapolis project uses a mobile network of smart inhalers to track when and where asthma attacks occur. City officials are now in the process of cross-referencing that data with maps of Louisville's tree cover, air quality, and heat island effect. "It would be ideal if we could understand where the tree canopy opportunities are as it relates to where some of our air challenges are," says Louisville innovation chief Ted Smith. That way, the city can identify asthma and allergy hot spots where some strategically planted trees might improve public health.

Louisville's Asthmapolis project uses a mobile network of smart inhalers to track when and where asthma attacks occur. (Photo by "WaveBreakMedia" via Shutterstock.com)

Louisville's data-driven pilot is the first of its kind worldwide, and it will take time to bring it up to scale. But cities can start by embracing asthma and allergy relief as a policy objective.

What Allergy Sufferers Can Do

The 2015 allergy season is shaping up to be particularly severe, according to Dr. Clifford Bassett, an allergist and immunologist who works with AAFA. "Because the winter precipitation was so robust ... it’s going to translate to very happy plants, trees, root systems, and soil," Bassett says. "Once the weather warms up, we’re going to see a very vigorous pollen surge that’s going to equate with pollen misery." To make matters worse, all those days spent cooped up inside might lead to what he calls a "double whammy" effect for people with indoor or year-round allergies: If your allergies aren't already under control, the extra spring pollen load could set you over the edge.

While you can't change the local pollen count, you can change your lifestyle to mitigate allergy symptoms. Dr. Bassett offers these recommendations for weathering the season:

Consult an allergist. A doctor can help you pinpoint what your allergic triggers are so you don't waste time and money on treatments that aren't right for you.

Pre-treat. Many allergy medications work better if you start taking them before symptoms occur.

A pair of sunglasses can help keep pollen out of your eyes. Putting your own hair between your shades and your face, however, is not necessary.  (Photo by "View Apart" via Shutterstock.com)

Limit your exposure. Pollen levels are higher on windy, dry, and sunny days, and lower on windless, wet, and cloudy days. Know the pollen count before you leave the house, and consider staying indoors if it's very high.

Don't line-dry your laundry. Pollen can collect on your clothes outdoors.

Use allergy-friendly houseplants. NASA has identified several houseplants that are great at removing indoor air pollutants. English ivy, ficus, bamboo palm, and peace lilies are among the best options.

Shower at night. Rinse pollen off your skin and hair before bed. This will help keep allergens off your sheets and pillows, where they can irritate you overnight.

Avoid cross-reactions. If you're allergic to seasonal tree pollen, you might experience mouth or throat tingling after eating almonds, apples, pears, and other fruits. This is the result of a cross-reaction between the fruit proteins and the tree pollen—another reason to see an allergist to find out precisely what's causing your symptoms.

Wear a hat and sunglasses outdoors. To help keep pollen out of your eyes.

Clean the air. Keep windows closed (in your car as well as at home), set your air conditioner on “recirculate,” and clean air filters regularly. Don't use fans that suck in outdoor air.

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