So you want to buy an indoor air quality monitor...
Now what?
Consumer grade indoor air quality monitors are becoming more common items in people’s homes and schools. What can you learn from them? What are their limitations? This is an attempt to answer some of these questions.
What are consumer-grade indoor air quality monitors?
What are consumer-grade indoor air quality monitors? Consumer grade indoor air quality monitors typically measure air concentrations of carbon dioxide (CO2), carbon monoxide (CO), particulate matter (PM), and/or volatile organic chemicals (VOCs). I describe these at consumer grade as they typically are less accurate, less precise and cost less than laboratory grade research equipment. Consumer grade indoor air quality monitors can range in cost from $50 to $500.
Does the reduced accuracy and precision matter?
Accuracy is the ability to hit a target. Precision is the ability to repeatedly hit the exact same spot (not necessarily the center of the target) over and over. Laboratory grade research equipment are both precise and accurate. Due to technology limitations most consumer grade indoor air quality monitors are not. But does make consumer grade indoor air quality monitors useless?
Not necessarily. Think about a thermometer. Some Laboratory grade research thermometers can accurately and precisely record temperature to the 0.001 of a degree Celsius. The thermometer in your car or outside your window may only be accurate to 0.1 degree Celsius, but that is good enough for you to get a good idea of what clothes to wear. Likewise, consumer grade indoor air quality monitors have limitations, but they can be an excellent educational tool to help us improve our indoor air quality.
Wait! What is “good” indoor air quality?
Often consumer grade indoor air quality monitor advertisements claim they report whether a room has “good indoor air quality”. So, what is “good indoor air quality?” This is a very difficult question to answer. Indoor air quality is generally defined as all the airborne chemicals, aerosols, and biological particulate matter that you can inhale while indoors. By good indoor air quality people generally are referring to air quality that has minimal or no health impact.
However, trying to describe indoor air quality with one parameter (PM, CO, CO2, VOC) is much like trying to describe the health of every person in the United States by how fast Tom, from Duluth, Minnesota, walks to the store on Sunday. You simply are missing too much information to make any declarations on the overall health of the population.
First, indoor air quality potentially encompasses tens of thousands of different chemicals and a multitude of aerosols of differing compositions. Measuring only one chemical or parameter (even total VOCs, described below) will not give a complete picture of the indoor air quality in a room.
Second, health impacts can vary widely. Carbon monoxide in high concentrations can be an immediate danger to life and health. In contrast, our bodies emit chemicals like squalene (skin oil) and isoprene (breath). So even those these chemicals maybe present in indoor air, they may not directly be a health concern (although squalene can react with ozone to create other chemicals that can be harmful at high enough concentrations).
Health impacts can also vary widely from one person to another. For instance, have you ever smelled an odor and then have another person say they don’t smell anything? This is because the concentration for someone to smell something (odor threshold) can vary by factors of ten. Health effects of chemicals in indoor air are similar.
Hence, describing “good indoor air quality” is a meaningless endeavor. We have tens of thousands of things in the indoor air we breathe (chemicals, aerosols, biological particulate matter) that have a wide range of health impacts in our air. We can’t describe our indoor air quality with a single parameter.
So, I would waste my money on an indoor air quality monitor?
NO! We will never fix all traffic problems in a city, but we can improve local traffic flow by improving intersections, adding mass transit or making cities more walkable. Similarly, we can use indoor air quality monitors to IMPROVE indoor air quality, at least for the one parameter we are measuring. And we can take advantage of our knowledge of indoor air quality principals to increase the likelihood that we are improving indoor air quality for things we aren’t measuring.
What are the four indoor air quality principals?
We can always improve our indoor air quality by taking four steps.
- Reduce Sources. Other than building materials of your home, you have daily control of many of the chemicals you bring into your home and emit into the air. Candles emit particulates and chemicals. Plug-in air fresheners emit chemicals that react with ozone. Scented dishwashing, laundry detergent and handwashing detergents put more chemicals into your indoor air.
- Increase Ventilation. In most locations, the concentration of chemicals indoors will be significantly higher than those outdoors (this may not be true in locations near chemical refineries, megacities with smog, large highways, or during wildfires). In most cases increasing ventilation will reduce the concentration of chemicals we have in our home. Ventilation can be as simple turning on an outside exhaust hood while cooking on the back burner of a stove or opening passive window vents. Newer homes have mechanical ventilation system that can run all the time.
- Add Filtration. Filtration can remove particles. Particles can carry less volatile chemicals around a home. Removing particles via filtration systems can reduce migration of these chemicals. Particle removal is a subject for other blog posts. I will just state that I prefer technologies that do not alter indoor air chemistry to remove particles (like HEPA filters).
- Control Moisture. Excess moisture can lead to mold growth. Mold can emit VOCs and exacerbate respiratory concerns (asthma and allergies). Enhanced ventilation should be done in a way that doesn’t lead to excessive water condensation.
What type of indoor air monitor should I purchase?
As mentioned above there are four major types of consumer grade indoor air quality monitors: carbon monoxide (CO), total volatile organic chemicals (VOCs), particulate matter (PM), and carbon dioxide (CO2). The following sections describe what you can learn from each type of monitor. Specifically, why we should worry about each chemical, where it comes from indoors, how the monitor work and how useful the monitors are.
What can I learn from a carbon monoxide monitor?
Why should I worry about carbon monoxide? Carbon monoxide is odorless and can be lethal in air at the 70 ppm to 400 ppm range. Homes with gas stoves can have carbon monoxide levels in the 0.5 ppm to 15 ppm range. Every year over 150 people in the US die from unintentional carbon monoxide poisoning.
How does carbon monoxide end up indoors at lethal levels? Carbon monoxide is the result of incomplete combustion. Typically, to reach lethal levels indoors a significant combustion source is needed. Often this is the result of faulty heating system, faulty gas stoves, vehicle or a portable generator operated in a confined space.
How do carbon monoxide monitors work? Carbon monoxide monitors are perhaps the oldest widely adopted consumer grade indoor air quality monitor and have been incorporated into fire alarms. Typically, the sensors use either a metal oxide or electrochemical sensor which changes current when carbon monoxide contacts the surface.
Are they useful? YES! Every home (especially those with a combustion source) should have a carbon monoxide monitor on the same level as any bedrooms. Consumer grade sensors are set to alarm at levels that are lower than health effects with a significant margin of safety.
What can I learn from a TVOC monitor?
What does TVOC mean? TVOC stands for Total Volatile Organic Compounds. VOCs are chemicals that contain carbon and oxygen atoms that will evaporate at room temperature. Perfumes and gasoline are two examples you likely have interacted with. TVOC has many different definitions depending on how VOCs are defined and how it is measured. Often VOCs are defined as a range of chemicals that have a defined set of boiling points or vapor pressures. Some laboratory grade instruments can identify and quantify the concentration of each chemical in the air sample in the range of chemicals described as VOCs. The TVOC value is then defined as the sum of all the chemical concentrations in this range. Other laboratory grade instruments will approximate the concentration of each chemical in the air sample in the range of chemicals described as VOCs. TVOCs values from these instruments are less accurate as they do not quantify each chemical with a standard of the same chemical.
How do they work? Consumer grade TVOC instruments are often based on metal oxide sensors. These instruments heat the surface of the sensor to oxidize the gas above onto the sensor surface. The oxidized layer changes the electrical resistance of the sensor. An algorithm is then used to convert this resistance to a TVOC concentration. Photoionization detectors work great when there is only a single chemical in the air and the algorithm is written specifically for that chemical.
Are there issues with consumer grade TVOC monitors? YES. In indoor air there are tens of thousands of chemicals that could get ionized to various degrees. Hence, the algorithm that the manufacture has used to calibrate your TVOC monitor may respond (or not respond) differently to chemicals in your house. Hence, due to accuracy issues consumer grade TVOC monitor values should not be compared with health metrics, other instrument types or other homes.
In addition to calibration and accuracy issues, the health impacts of the same TVOC reading can vary widely. Let’s say an instrument reads 10 ppb (10 VOC molecules per 1 billion total air molecules). Hypothetically, there could be a case in a garage where that reading consists of 9 ppb benzene (a concentration nines time greater than the OSHA short term exposure limit for a known human carcinogen) and 1 ppb of other chemicals. In a crowded house, that same 10 ppb reading could consist of 2 ppb of isoprene, 2 ppb of ethanol, 2 ppb of acetone, 2 ppb of methanol and 2 ppb of other chemicals. What do all those chemicals in the house have in common? They are emitted from human breath. Hence, the health concern for identical readings maybe completely different. This is why TVOC readings from consumer grade monitor should not be used to declare if a space is “healthy” or has “good indoor air quality.”
Are TVOC monitors useful? YES! If you’ve got to this point, you may be surprised that I still think TVOC monitors can be useful. That’s because the CHANGE in a TVOC value can still give you valuable information. You can see the impact of various activities on your indoor air quality. You can see the impact of plugging in an air freshener versus hanging laundry that has been washed with scented detergent. If you keep in mind that only the change in values matters in one environment, then seeing which of your activities you might want to change to improve your indoor air quality is a valuable use of a TVOC monitor. You don’t have to rely on some so-called “expert” with a blog on the internet, you can see how YOUR actions impact YOUR indoor air quality.
What can I learn from particulate matter monitor?
What is particulate matter? Particle matter consists of things that float in the air. These can be liquids or solids that are bigger than individual molecules. Particles can run into each, sticking and grow in size. If they are large enough (and many are not) they can settle on to horizontal surfaces (dust). Often particulate matter is also referred to as an aerosol (we are going to use particulate matter in this document to be consistent with the instrument descriptions).
Particulate matter is tremendously diverse. Particulate matter can come from a wide range of sources, including combustion (gas stoves, gas furnaces, cars), evaporate of water (sea spray, sonicating humidifiers), and chemical reactions (ozone is a driver of indoor chemistry).
The size of particulate matter can range five orders of magnitude (a factor of 100,000) from 1 nanometer to 100 microns. Particulate matter larger than 100 microns will be visible and drop in seconds (hair is about 70 microns in diameter). Most particle matter smaller than 10 microns will not drop indoors and be removed as air leaves an indoor space. For perspective, a blue whale is only 1,200 times large than a hummingbird.In addition, particulate matter can vary in content. It may have the density of brick or the density of a pillow stuffing. Particulate matter may be as reactive as mentos and coke, or it may be as benign as butter.
Why does it matter? Particulate matter can impact your heart, lungs and enter your blood stream. Smaller particles (less than roughly 4 microns) penetrate deeper into the lungs. Premature death, heart attacks, aggravated asthma, irritation, coughing and difficulty breathing have all been linked to elevated particulate matter concentrations.
Outdoor PM2.5 (particles smaller than 2.5 microns) is measured in real time and available online in the United States. There are also health-based standards for outdoor PM2.5 concentrations (United States annual average of 15.0 µg/m3; 24-hour of 35 µg/m3). Due to the analytical issues with consumer grade particulate matter sensors (see below) direct comparison to the health-based standards is not advisable.
Most consumer grade monitors will report PM2.5 values. This means they are reporting on particles smaller than 2.5 microns. To do this the PM2.5 monitors often drawn air in with a small pump. The air then goes through one or more turns that cause larger particles to hit the side of the turn, much like when a car goes around a turn too fast and runs off the road. The smaller particles can make it through the turn.
Are there issues with consumer grade particulate matter monitors? The type of particle will influence how much light gets scattered. An outdoor particle near the ocean (salt based) will scatter light in a different manner than a particle from frying bacon or from a wildfire. Different light scattering can result in different reported particulate matter concentrations for the same mass of particles in the air. Since most particulate matter monitors are calibrated to outdoor particles, the values they read indoors can be off by a factor of three.
In addition, light scatter particulate matter monitors are limited by the wavelength they use to detect particles. Consumer grade particulate matter monitors will not be able to detect particles smaller than 0.3 microns. Gas stoves, candles and chemical reactions can all produce particles smaller than 0.3 microns. These “ultrafine” particles can penetrate deeper into the lungs potentially causing respiratory issues. Hence, consumer grade PM2.5 meters are reporting concentrations for particles between 0.3 to 2.5 microns. Hence, a low reading by a consumer grade monitor does not mean there is a no concern regarding ultrafine particles.
Some particulate matter monitors that use nephelometer sensors claim to be able to measure varying “bins” of particles sizes (PM10, PM2.5, PM1). These instruments use algorithms based on laboratory measurements to assign some of the responses to various size fractions. However, the particulate matter in your indoor air is unlikely to be similar to the laboratory particulate matter. Hence, the algorithms for various size fractions are unlikely to be applicable to your indoor air.
Due to these issues, consumer grade PM2.5 values may or may not be comparable to health metrics. A high consistent PM2.5 reading is a reason to investigate the environment further with higher quality instruments, but the reading may not be cause for alarm.
Are particulate matter moniotors useful? YES! Just like with TVOC the CHANGE in a PM2.5 value still gives you valuable information. Spikes in PM2.5 readings are great opportunities to see the impact of various activities and remedial actions on your indoor air quality. How bad is frying bacon versus cooking it in an oven? How much does opening various windows help reduce the PM2.5 after cooking bacon? How much does turning on a vent fan help? Is bacon worse than the toaster or a candle? Note that all candles emit particulate matter. It’s just some will emit particles smaller than a consumer grade sensor can detect.
Warning: Particle matter monitors will not help identify if there is lots of viruses in the air. Trying to see an increase in particle counts due to virus emissions from humans is like trying to see the increase in the water depth in a pool during a light rainstorm. There are just not enough virus particles in the air relative to the particulate matter already present.
Remember, the goal is not to eliminate every single particulate matter from indoor air. But rather figuring out how to do activities that we love in a manner that reduces our exposures. For instance, blowing out birthday candles outdoors, or cooking on the back burner with the vent fan on.
What can I learn from carbon dioxide monitor?
What is carbon dioxide? Take a deep breathe. Your lungs exchange oxygen in air with carbon dioxide. The air you breath in is about 0.04 % carbon dioxide. The air you breath out is about 4 % carbon dioxide. Importantly, the amount of carbon dioxide that we emit depends on our activity level (sleep versus a spin class) and our size (babies emit less).
Why does carbon dioxide matter? Given its relatively high concentration compared to other chemicals and emissions from all people, carbon dioxide is a good proxy of emissions from humans. In fact, we can use carbon dioxide as a metric of how much air you’re breathing in from other people in the room. If people are the only source of carbon dioxide in the room you can determine the fraction of air you are breathing that has been in other people’s lungs.
For instance, if the indoor carbon dioxide concentration is 800 ppm (800 carbon dioxide molecules per 1 million total air molecules) and the outdoor concentration is 420 ppm, then 1 % of every breath a person takes in the room will consist of air that has been in someone’s lungs. If the indoor concentration is 2000 ppm, then the rebreathed fraction will be over 4 %. If someone in the room is emitting viruses that are airborne in their breath, then we want a lower rebreathed fraction to lower our risk of getting sick. We can lower the rebreathed fraction by reducing the number of people in the room or increasing ventilation (outside air will dilute the indoor carbon dioxide concentration). We can also decrease our risk of infection via aerosolized viruses by filtration, but this will not lower the rebreathed fraction as filters do not lower carbon dioxide concentrations.
What do humans emit in addition to carbon dioxide? Our breath contains a variety of organic chemicals (including isoprene, acetone, methanol and ethanol), inorganic chemicals (water, salts) and biological agents (including viruses). These emissions can be associated with aerosols or particulate matter in our exhaled breath. Our skin can emit odor causing chemicals (including ammonia).
Hence, carbon dioxide can be a proxy (but not a direct measurement) for risk associated with viruses or complaint-causing odors. Importantly, just because the carbon dioxide concentration is high indoors that does not mean there is virus or odor in that space, rather there is an increased risk that virus or odors are present.
How is carbon dioxide measured? Carbon dioxide is typically measured in consumer grade instruments with a nondispersive infrared sensor (NDIR). The infrared light passes through the air and carbon dioxide molecules absorb a specific frequency. A detector determines the fraction of that specific frequency of infrared light that has passed through the sample. The fraction that passes through the sample is proportional to carbon dioxide concentration. Cheaper non-NDIR carbon dioxide consumer grade monitor use sensors that measure other chemicals and then try to correlate that response back to carbon dioxide. Since this correlation is not universal, these sensors can have large errors.
Are there issues with consumer grade carbon dioxide monitors? Consumer grade carbon dioxide monitors are sensitive to relative humidity extremes. In addition, single beam NDIR are subject to more drift over time than dual beam monitors. The accuracy of NDIR sensors is about ± 50 ppm or 2% to 3% of the reading (whichever is greater). Hence, a reading of 801 ppm is really no different than a reading of 751 ppm or 849 ppm.
Many monitors have a built-in auto background calibration feature (ABC). This feature assumes that at some point over a set period of time (e.g. a week) there are no people present and the monitors is exposed to outside air equivalent concentrations (400 ppm to 420 ppm). The monitors then resets the lowest reading over the time period to 400 ppm to 420 ppm and adjusts the calibration accordingly. If a carbon dioxide source (combustion, people or pets) are always present in the room where the monitor is monitors with auto background calibration will slowly drift upwards.
There are other potential sources of carbon dioxide indoors. Any combustion source can elevate the reading. A gas stove can elevate readings. To counter this, make sure if you are trying to determine the impact of human emissions that measurements occur when indoor combustion are off.
These calibration schemes and other factors (like dust building up on the sensor) mean that carbon dioxide monitors can drift over time. In addition, outside combustion sources, like roadways or idling vehicles, can elevate the readings. As a result, I recommend using the difference between indoor and outdoor readings, rather than the direct reading, as proxy for human emissions.
Indoor carbon dioxide concentrations can be used in three ways. First, they can be used to see the impact of ventilation actions on indoor air quality. Assuming carbon dioxide sources indoors are constant, the relative impact of turning on exhaust fans, open windows or doors will be reflected in the indoor carbon dioxide concentrations. The impact of ventilation actions will vary depending on the outdoor temperature, wind speed and direction.
Second, several organizations and some carbon dioxide monitors have “stoplight” values for carbon dioxide concentrations (CDC, Belgium). Some of these stoplight values are based upon the rebreathed fraction and the relative risk for airborne transmission of viruses. In general, these guidelines recommend keeping indoor carbon dioxide concentrations below 900 ppm to 1,200 ppm. These values translate to 500 ppm to 800 ppm above the outside concentration. Given the drift uncertainty of most carbon dioxide monitors and varying outdoor concentrations, determining the difference between the indoor and outdoor concentrations will give a more accurate value to compare to guidelines (i.e. 500 ppm to 800 ppm above the outside concentration).
If “stoplight” method is being used to estimate ventilation in a classroom or building, the monitor should be placed in the room for the entire day with normal number of occupants performing typical activities. This is because it takes time for carbon dioxide concentrations from human emissions to build in a room. The daily peak concentration, or the concentration at the end of the day should be compared to guidelines. In addition, the room should be checked when weather, occupant density and season change as all will impact the ventilation rate. I discussed this proces in detail here.
The third way carbon dioxide monitors can be used in indoor spaces is to estimate the air change rate (λ). The air change rate is the rate at which air enters a space divided by the volume of the space. For instance, if the air change rate is 1 per hour, this means a volume of air equal to the room volume will enter the room in 1 hour. Importantly, this does NOT mean that a contaminant (or virus) will leave the room in 1 hour. Rather, due to the mixing of the air it will take close to three divided by the air change rate (3/ λ ) to remove 95 % of the contaminant from the room (assuming a contaminant that does not settle or react). So, for an air change rate of 6 per hour, it will take 0.5 hours (30 minutes) to remove 95 % of the contaminant.
How do we use carbon dioxide monitors to determine air change rates? We elevate carbon dioxide concentration in a space, leave the space and record how fast the concentration declines over time. With some math we can estimate the air change rate from the rate of carbon dioxide concentration decay. Since carbon dioxide is non-reactive and moves with the air, it serves as a tracer chemical. These are the steps to conduct an air change rate experiment using a carbon dioxide monitor:
- Place carbon dioxide monitor outside. Allow it to equilibrate for 10 minutes. Record value.
- Enter space. Place carbon dioxide monitor in the middle of the room.
- Increase carbon dioxide concentration via natural (e.g. several breathing while vigorously dancing or exercising) or assisted means. Assisted means that have been used include carbonated drink or paint ball carbon dioxide cannisters, melting dry ice, or baking soda and vinegar volcanoes. Wait until the carbon dioxide concentrations exceeds at least 1,000 ppm. Concentrations higher than 2,000 ppm are unnecessary, and some monitors can’t record above this value. Carbon dioxide can start to be dangerous above concentrations of 5,000 ppm for 8 hours and immediately endangers to life and health at 30,000 ppm for short term exposures (e.g. 15 min). Ensure all carbon dioxide sources are off (e.g. stoves) during this time.
- Have all people leave the space for 30 min to 2 hours. Make sure the monitor is recording the concentrations that can be downloaded later or view the monitor through an exterior window and manually record the concentration every] one to five minutes.
- Remove monitor from space. Again, place the carbon dioxide monitor outside. Allow it to equilibrate for 10 minutes. Record value.
Once we have the data, we can apply a mass balance and some math to determine the air change rate. A mass balance is simply a way of counting things. For instance, if we wanted to know the number of people in a store over time we could constantly run around the store and try to count everyone in the space. Or we could stand at the door and count the number of people entering and leaving the store over time with the difference being the number of people in the store at any given moment. A mass balance on the carbon dioxide concentration in a room works the same way as counting at the door. This spreadsheet uses a mass balance approach to estimate the air change rate in a single room building (like a portable classroom).
A home can have an air change rate of 0.1 per hour to 1.0 per hour, depending on the leakiness of the exterior surfaces (e.g., walls, windows, roof, floor), the operation of ventilation equipment indoors, indoor temperature, and weather. A commercial space may have an air change rate of up to 5 per hour. Some areas of hospitals may have air change rates as high as 12 per hour. Classrooms typically are designed to operate at 2 per hour to 3 per hour, but during the pandemic several organizations recommended up to 4 per hour to 6 per hour.
WARNING: There are several cautions when using the carbon dioxide decay approach to estimate air change rates. First this method is a snapshot in time. Air change rates can change by up to a factor of 2 to 5 depending on the weather and building operation. Second, the curve fit used in this approach will introduce uncertainty. Combined with the dynamic nature of buildings means the air change estimate should only be reported to one significant figure. Next, air change based on carbon dioxide only accounts for outside air being brought into the space. It does not account for recirculation of air that may have aerosols (e.g. viruses) removed via central or portable filtration systems. The effective air change rate for aerosols (viruses) that includes these loss mechanisms will likely be higher than the value determined with this method. In addition, this method assumes during the decay phase the entire building is well mixed and all carbon dioxide sources are out of the building. This is likely not true in buildings such as schools. Finally, high air change rates do not guarantee healthy indoor air. Outdoor air may have unhealthy constituents. Virus transmission can still happen in rooms with higher air change rates, although risks in these rooms is lower than rooms with lower air change rates.
Given the uncertainty associated with estimating air change rates with this decay rate method, lower air change rates should be used to start discussions with building facility managers on how to improve indoor air quality.
If you are tired of reading, you can find my presentation on limitations of using consumer grade carbon dioxide monitors to assess outdoor ventilation here.
Great, so now which indoor air monitor should I purchase?
I do not recommend specific indoor air quality related products, so I can remain impartial in my day job. But I will share that I have two carbon dioxide monitors as I am currently interested in ventilation in rooms with lots of people (e.g. schools). If I lived in a place frequently impacted by wildfires I would likely get a PM2.5 monitor.
Here are some things to keep in mind when you are looking for consumer grade indoor air quality monitors: Does it display a number that is visible on the insturment or is an app needed? Can the data be logged? Does the system log data to the web? Being able to see or manipulate the data makes these instruments more useful in evaluating how your actions impact the indoor enviornment.
What is the bottom line?
Consumer grade indoor air quality monitors are a great educational tools. While they are not great at assessing whether the indoor air is “healthy,” they give actionable information on how your actions impact the dynamic nature of an indoor environment. For instance:
- What is the impact of turning on your kitchen exhaust fan when cooking?
- Can you reduce your PM2.5 concentrations by cooking on the back burner?
- Which is worse - frying bacon or using a toaster (PM2.5)?
- Is cooking with peanut oil better than olive oil with regards to PM2.5 concentrations?
- What happens to the PM2.5 concentration if you use your fireplace?
- Does vacuuming impact the PM2.5 concentration?
- Is the air change rate higher on a windy day?
- How much does opening a classroom’s windows impact carbon dioxide concentrations in the fall? In the winter? How many windows need to open? How much? Does opening the door help or hurt?
- How high does the carbon dioxide get when you have guests over for a party? How many open windows are needed to keep the concentrations below 1000 ppm?
- How good is the ventilation in your grocery store (carbon dioxide with stoplight approach)?
The list of indoor air issues to investigate with consumer grade indoor air quality monitors can be as long as your imagination makes it. Get out there an investigate your world!