Hay fever season and the ragweed allergies it brings may be getting more intense and lasting longer, according to a 2011 study from the U.S. Department of Agriculture (USDA) published in the Proceedings of the National Academy of Sciences of the United States of America. "The main takeaway from the study is that we are seeing a significant increase in the season length of ragweed and this increase is associated with a greater warming at northern latitudes, consistent with projections regarding climate change," said Lewis Ziska, PhD, the lead author of the study and plant physiologist with USDA's Crop Systems and Global Change Lab. (The study found that the length of the ragweed season increased by as much as 27 days between 1995 and 2009 in various areas of the U.S.).

Climate change threatens human health in a number of ways, but allergies may be the most immediate, easy-to-recognize ailment, said Linda Marsa, investigative journalist and author of the book Fevered: Why a Hotter Planet Will Hurt Our Health—And How We Can Save Ourselves. And the problem isn't only longer allergy seasons.

Under normal circumstances, a single ragweed plant creates one million pollen grains. But in an environment with more carbon dioxide (CO2)—the main driver of climate change—plants produce three to four million pollen grains, explained Clifford Bassett, MD, medical director of Allergy & Asthma Care of New York and a member of the public-education committee at the American College of Allergy, Asthma & Immunology. But the problem doesn't stop there. CO2 in the atmosphere is like plant food for weeds, causing them to produce pollen that contains more allergenic proteins than normal, said Marsa.

A study in the journal Environmental Health Perspectives found that triclosan (an antibacterial chemical used in toothpastes and other personal care products) may play a role in worsening allergies. The study researchers looked at 3 years of health data from about 5,000 people and found that urinary levels of triclosan were linked to allergies and hay fever. This finding supports the "hygiene hypothesis"—sanitizing our homes and environments creates a weaker immune system that is less able to respond to bacterial and viral threats.

"Hygiene can protect us from infections," said Erin Rees-Clayton, PhD, a study author and a research investigator at the University of Michigan School of Public Health. "But some of the chemicals in hygiene and cleaning products may have more risks than benefits."

Researchers at the Columbia Center for Children's Environmental Health linked childhood exposure to BPA (bisphenol A, used in plastics, the lining of food cans and thermal receipts) to asthma.

"Asthma prevalence has increased dramatically over the past 30 years, which suggests that some as-yet-undiscovered environmental exposures may be implicated," said study leader Kathleen Donohue, MD, assistant professor of Medicine at Columbia University Medical Center. "Our study indicates that one such exposure may be BPA."

Donohue and her colleagues analyzed health data from 570 pregnant women and their children at ages 3, 5 and 7. They found that exposure to BPA in early childhood—even very low levels of it—increased the risk of asthma in the children.

What are the jobs most likely to give you asthma? That's the question posed by researchers at the Imperial College London, who studied nearly 10,000 people to see which careers were most likely to trigger asthma. Of the 18 asthma-producing occupations, seven of them involved regular use of cleaning products.

That's not too surprising when you consider that 53 percent of cleaning products damage the lungs, according to an analysis by the Environmental Working Group. (And "green" cleaning products are not necessarily free of lung-damaging compounds).

A 10-year study of 3,000 children found that those with vinyl flooring in their bedrooms were one and a half times more likely to have asthma than children with wood, linoleum or other flooring materials. (The vinyl is polyvinyl chloride or PVC, a type of plastic widely used in construction). If mothers had vinyl flooring in their bedrooms while pregnant, the children were twice as likely to have asthma. "Our results suggest that PVC flooring exposure during pregnancy could be a critical period in the development of asthma in children at a later time," concluded the Swedish researchers in the International Journal of Environment and Health.

The researchers speculate that it's the phthalates—chemicals used to soften plastic—that are doing the dirty work. Phthalates leach into household dust, creating constant exposure.

You've probably never heard of it, but in 2013 the dermatologists of the American Contact Dermatitis Society dubbed it the "Allergen of the Year." The compound? (Take a deep breath). Methylisothiazolinone (MI)—the preservative in most premoistened toilet, feminine and baby wipes (and in various liquid soaps, hair products, sunscreens, cosmetics, laundry products and cleaners). MI replaced other toxic, irritating preservatives, such as formaldehyde and parabens. The only problem: MI is irritating, too!

"In the last 2 or 3 years, we've seen a big increase in people with allergy to MI," said Matthew Zirwas, MD, director of the Contact Dermatitis Center at the Wexner Medical Center of the Ohio State University. The allergy produces red, raised, itchy bumps similar to poison ivy. The three areas most affected by the rash include the fingers and hands (from handling wipes), the buttocks and genitals (from applying the wipes) and the face (from soaps and shampoos). Needless to say, most people (and their primary care doctors) never figure out the cause of the rash.