Frequently Asked Questions

Who is PurpleAir and how did this get started?

We are a grassroots group with a technical background that wanted to measure the pollution coming from a mining operation in our backyard. The website and map was started in November 2015. Sharing the data and our research with others created a need for more monitors, to expand the network and to test the sensors by collocating them with other monitors.

What do the numbers on the PurpleAir map mean?

The PurpleAir Map displays the points using the Federal Environmental Protection Agency (EPA) Air Quality Index (AQI) scale. The AQI allows comparison for different pollutants with an easy to visualize color scheme. The air goes from good green air to yellow to orange to red to purple air, or even hazardous maroon air. The graphs give further information on different counts, relative weights and sizes of particles in the air.

Is PurpleAir a non-profit organization?

PurpleAir has not applied for non profit status for a number of reasons. We currently operate under an llc.
We reached this point by using our own resources, some donations and help from friends. Due to numerous requests for sensors, in late February 2016 PurpleAir accepted pre-orders of a small batch of prototype sensors.

How do PurpleAir sensors work?

They use laser beams to detect the particles going past by their reflectivity, like dust shimmering in a sunbeam. The PM 2.5 and PM 10 micro-gram weights are calculated from the counts. The values are averaged every 20 seconds and graphically displayed on our website.

What pollutants does PurpleAir Monitor?

PurpleAir displays the particulate matter numbers, in both the PM 2.5 and PM 10 range, which are criteria pollutants. The Federal and State air quality alert system uses particulate matter numbers and ozone to warn the public. Ozone sensors may be displayed soon, as we are just now doing experiments with a version that could be deployed.

How do PurpleAir sensors compare to others?

PurpleAir sensors read very similar numbers to other sensors of the same type (laser counters). During a recent inversion, PurpleAir sensors were reading correlated but slightly lower than University of Utah sensors on the MesoWest map. Some sensors use very different methods to measure particles and so we expect some disagreement with their numbers. It is the amount of disagreement we are trying to figure out and will let you know when we have more information from the University of Utah and SCAQMD tests.

How does PurpleAir decide where to place sensors?

Every care is taken to place monitors in locations that are representative of the air we breathe every day. They are placed in neighborhoods, on the side of houses, a few feet above our heads. They use the homwowner’s Wi-Fi and power. Local pollution like cigarette smoke, BBQ’s, fireplaces and idling cars can cause spikes in the short term graphs.

What kind of quality control measures does PurpleAir have?

Quality techniques like an initial decay test, round robin placement, and correlation testing with new and older monitors are used. One of the issues that faces these devices, and others, is what is called “drift”. This happens with optical particle sensors because of dust that may settle inside the device, and will give lower readings over time. A maintenance schedule is necessary with any measuring device, but ours are relatively easy to maintain.

Does PurpleAir make indoor sensors?

PurpleAir sensors can be used indoors, with or without the outdoor housing. We provide a link to view your data separate from our outdoor map. There is a demand for indoor monitoring, and it gives us a reason to develop a version with a graphic display. We are testing display screens and housing prototypes for possible indoor versions.

How can I get PurpleAir data?

The simplest way to view our data is the PurpleAir Map. Our data is also included on the University of Utah's MesoWest Map (we are the triangles). We frequently share raw data with researchers and others interested in looking at it. If you want access to data for whatever reason, please feel free to contact us and explain your need.