Most of the plots in CHATTER are heat maps. A heat map is a grid of squares where the number in each square is the measurement, and the color of the square tells you at a glance how that number compares to a health-based limit.

Each row is an air pollutant (or, on some tabs, a health effect or a total). Each column is a period of time — a calendar year, the most recent 365 days, or the most recent month.

The number in each square is a multiple, written with an “x.” It is the measured level divided by the limit. So 1.00x means the level is exactly at the limit. A number below 1.00x means the level is below the limit — for example, 0.5x is half the limit. A number above 1.00x means the level is above the limit — for example, 3.00 means the level is three times the limit, and 6.50x means six and a half times the limit.

The color legend beside each plot shows what those multiples mean. The palest color means the level is below the limit. The darker the color, the further above the limit the level is. The label on the legend sits at the line where each band begins, so a square colored in the band starting at “5x” is somewhere between five and ten times the limit.

Squares shaded gray with diagonal hatching mean there is no measurement for that pollutant in that period — usually because the monitor does not measure it. A gap in the data is not the same as a low reading, so CHATTER marks it differently rather than leaving it blank.

The bottom row of some plots is a Total. It adds up the individual pollutants above it, to show the combined burden for that time period.

For a worked example using real numbers, open the “How to read this plot” panel at the top of any of the plot tabs.

An air pollution monitor is an instrument that measures the pollutants in the outdoor air to understand air quality conditions. See EPA’s overview of ambient air monitoring.

An EPA screening level is a reference point, based on scientific studies, that helps identify when air pollution may be high enough to raise health concerns and warrants added action or investigation. CHATTER compares measured air pollution levels to these screening levels and shows when pollution levels exceed them.

Depending on the amount by which a pollutant exceeds the screening level, further action may be warranted to reduce people’s exposure to unhealthy levels of outdoor air pollution. For measurements falling below the level, no further action may be warranted. Exceeding a screening level does not mean that health effects will occur. Instead, it indicates that the pollution level may require closer evaluation and, in some cases, actions to reduce exposure.

EPA provides screening levels for both cancer risk and other health effects, such as breathing problems, heart disease, and other chronic health conditions. CHATTER compares yearly air pollution levels with the EPA screening level.

EPA uses past scientific studies to understand cancer risks related to air pollution levels over a person’s lifetime. EPA generally considers a lifetime cancer risk of one additional cancer case per one million people exposed over 70 years to be a low level of concern. Risks above this level suggest closer evaluations are needed, while risks approaching 100 additional cancer cases per one million people exposed over 70 years are generally considered a greater concern and may require actions to reduce exposure.

A lifetime cancer risk of one in a million means that if one million people were exposed to the same level of a pollutant continuously for 70 years, one additional person may develop cancer because of that exposure.

CHATTER uses EPA’s one-in-a-million increased cancer case air pollution screening levels for residents. In the heat map key, the white color means the air pollution level is equal to the one-added-cancer-case-per-million-people screening level. The darker the color, the higher the value compared to the screening level threshold. Air pollution levels 100x the screening level are shown in the darkest orange. The 100x screening level means increased risk of 100 cancer cases in one million people if they were exposed over a 70-year lifespan.

The EPA also sets screening levels for long-term health outcomes that are not cancer. EPA uses scientific studies to set screening levels for air pollution levels that may pose a risk of long-term health problems other than cancer, such as breathing problems, heart disease, and other chronic health conditions. Air pollution levels are acceptable when they are equal to or below the screening level, and suggest further action when they are ten times greater than the screening level.

National Ambient Air Quality Standards (NAAQS) are air quality standards set by EPA under the Clean Air Act for common pollutants: ozone, fine particles, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These standards are designed to help protect public health, including for ensitive groups such as children, older adults, and people with heart or lung disease. Air pollution measurements from monitoring stations are compared to these standards to determine whether an area’s air quality meets federal health-based requirements.

No. Some air monitors measure only a few air pollutants compared to others. The state and EPA decide which instruments are placed where and which pollutants they will measure. You can find a detailed list of all the air monitoring sites in Texas and what they measure on the TCEQ website. You can also find ones that are operated in collaboration with the federal government on the EPA AirNow website.

CHATTER includes air pollutants measured at local air monitors for which EPA has published a health screening level and information on the body system or health outcome that is affected by the pollutant. Examples of health systems or outcomes the pollutant impacts are cancer, heart, lung, liver, and immune system. CHATTER also includes the six common pollutants regulated by the Clean Air Act.

There are several reasons why an air pollutant may not be in CHATTER.

CHATTER includes only hazardous air pollutants that are measured by specialized air monitors called AutoGCs. These monitors continuously measure a specific group of air toxics for which EPA has published health screening levels. Some hazardous air pollutants are not included because they are not routinely monitored by the state, or because they do not have the information needed to evaluate potential health impacts.

CHATTER also only includes pollutants where at least 75% of the measurements in a year were recorded. Sometimes a monitor has data missing over time. When more than 25% of the measurements are missing, this can lead to error or too much uncertainty about the measurements that were not included.

It depends on the group of pollutants, because EPA compares each group to its standard in a different way.

For the cancer risk pollutants and the other health risk pollutants, CHATTER uses the yearly average concentration to approximate the level of pollution people are exposed to over a longer period. This is a health-protective way to summarize a year of monitoring, and it is consistent with EPA methods for long-term exposure.

For the EPA criteria pollutants, an annual average is not the right comparison. Each National Ambient Air Quality Standard (NAAQS) is written for a specific averaging time and a specific statistic. CHATTER uses the same form EPA uses for each pollutant. This is a statistic called the EPA Design value which is defined below.

A design value is the specific statistic EPA uses to compare a monitor’s measurements against a National Ambient Air Quality Standard. The Clean Air Act specifies acceptable levels of pollution defined by concentration or amount of pollutant measured over a specific length of time. They might use the third highest daily average value over a 3 year period by location for fine particulate matter (PM2.5). Whereas, ozone thresholds may be based on highest average levels measured over an 8 hour period in the last year. Each criteria pollutant has its own averaging time and appropriate statistic, so a simple yearly average would not be comparable to the standard. CHATTER calculates each pollutant using the form EPA specifies:

Each cell in the criteria pollutant heat maps shows the design value divided by the standard. A value below 1.00x means the pollutant is within the standard; a value above 1.00x means it is above the standard.

One important limitation. EPA’s official design values, the ones used to formally determine whether an area meets a standard, are averaged over three consecutive years of monitoring. CHATTER calculates each year separately, so the yearly columns show a single-year design value. This makes year-to-year changes visible, which is the point of the tracker, but it means a CHATTER value above 1.00x is not the same as an official EPA finding that an area has violated the standard. Treat it as a signal worth a closer look, not as a regulatory determination.

Details of regulatory limits for criteria pollutants are given by the EPA NAAQS Table.