Ever noticed a slight “purple haze” descending on your town due to colored streetlights? From Reddit to Quora, the internet is curious what’s going on. No, it’s not some Jimi Hendrix tribute you missed. It’s not even special holiday lighting your city is putting up to celebrate Halloween. It’s caused by a defective coating peeling off LEDs imported from China. It’s certainly confusing drivers across the country, and it may be impacting safety.

Here are the details: LEDs are much brighter than traditional incandescent bulbs. They also use 1/4 the electricity and can last 14 years. Sound like a win-win-win? Well they have one downside. Instead of the clear white light of the sodium lamps traditionally used for street lights, LEDs only come in various colors. That’s right, manufactures can engineer LEDs in a variety of colors, but not pure white.

So how do you make an LED that appears light? As with many things, there’s an expensive and durable way and a cheap but less durable way. The hard way is to build a fixture that is a mixture of diodes emitting red, green, and blue light (or red, yellow, and blue) in ratios engineered to appear white. The cheaper way is to take a single blue diode and coat it in phosphorous. The phosphorous will absorb blue wavelengths while allowing red and yellow to slip through. The resulting mixture appears white…at first.

If this phosphorous layer peels off (in a process called delamination) more of the blue light escapes. This blue light mixes with the red light the surviving coating produces and the result appears purple. Remember I said that an LED can last 14 years? Well this phosphorous layer may not. In fact, some of the ones used in street lights are deteriorating far ahead of schedule–and many come from the same company.

Like many industries, street light manufacturing has consolidated over the years. A company called Acuity Brands dominates the market. Many of the cities and towns suffering from purple street lights admitted to journalists that they bought them from Acuity. And Acuity, in turn, reports it sources its lights from a vendor in Asia and struggled with a bad batch of LEDs between 2017 and 2019. As early as 2020, the phosphorous layer began peeling off these LEDs, turning street lights purple.

According to Scientific America, these purple street lights could be dangerous for both drivers and pedestrians. They may not be as bright once the phosphorous has peeled off. The tint also makes it harder to see detail and distinguish between different colors. So if the streetlights in your town have been turning purple, make sure to pay extra attention on the roads.

Ram Seshadri is a materials science professor at the University of California, Santa Barbara. He worries that the widespread “purpling” of Acuity street lamps may turn folks off to LEDs. But he hopes it doesn’t.

“The materials that are normally used to make the phosphor are extremely stable…There must be some issue with the manufacturing or some particular LEDs, because most LEDs that the world is using don’t face this problem.”

Ram Seshadri, professor of materials science at the University of California, Santa Barbara