Property owners or managers who are handed their test results for indoor air contaminants likely won't stop to wonder how you got these results, but industrial hygiene technicians know how important testing methods are to capturing accurate results. Passive and active air sampling are both methods of collecting air samples. The difference between active and passive air sampling is in the way you obtain those samples.
Laboratory studies have not demonstrated gross differences between passive and active samplers for gases and vapors, and this has been confirmed in the majority of field studies. So, how should you choose between these sampling methods? In this guide, we'll compare passive and active air sampling methods so you can determine the best choice for your application.
Table of contents:
- Passive Air Sampling
- Types of Passive Equipment
- Active Air Sampling
- Types of Active Air Sampling Equipment
- Shop Online With Zefon International
Passive Air Sampling
Passive air sampling is a method primarily used to sample for gases and vapors. In this case, passive air sampling is more accurately termed "diffusive sampling" because it relies on the natural process of diffusion. This method may be deemed "passive" because, unlike active sampling, it does not involve pumping air to collect it.
Diffusion samplers come in various types, but they all have the same basic design. At one end of the container is the diffusive surface, where gaseous and vapor molecules enter. At the other end is the sorbent medium, where the molecules are collected. Each analyte — that is, a substance being measured — has its own diffusion coefficient, or uptake rate. With this information combined with the amount of exposure time, you can determine a time-averaged sample concentration for the analyte. Diffusive sampling works especially well for long-term testing.
Some of the reasons to choose diffusive air sampling include its:
- Cost:Diffusive air sampling uses minimal testing equipment that is especially affordable. Because of the low cost involved, passive air sampling may be a more realistic choice for conducting ongoing air monitoring.
- Convenience: This testing method is also convenient since correctly setting up the testing equipment requires very little expertise, and the testing process doesn't require any supervision or monitoring.
Diffusive air sampling also comes with some disadvantages:
- Limited to gases and vapors: If you need to sample aerosols and particulates, diffusive sampling is not an option since particles do not follow the same principles of diffusion as gases and vapors do. You must use active sampling to sample for particulates and aerosols.
- Few validated government methods: The National Institute for Occupational Safety and Health (NIOSH) has only two validated methods using diffusive samplers, toluene (Method 4000) and nitrogen dioxide (Method 6700). However, the Occupational Safety and Health Administration (OSHA) has many more methods — for example, Method 1014 for styrene. Manufacturers have validated methods, as well.
- Invariable uptake rate: Unlike active sorbent tube samplers, diffusive samplers have an uptake rate fixed by design. An uptake rate that is too high can lead to overloading at high concentrations, while an uptake rate that is too low may not collect sufficient sample for analysis when concentrations are low.
Types of Passive Equipment
Diffusive equipment is divided into three main types, all having a diffusion path:
Each of these designs has a different surface area and diffusion path, which affects the device's uptake rate. Tube-style devices have relatively small openings and long diffusion paths. Therefore, they tend to have the lowest uptake rates. In contrast, radial-style samplers' porous cylinders make for the highest uptake rates. Badge-style devices generally lie in the middle. Badges are also available from the same manufacturer with different uptake rates to accommodate different expected concentrations.
These devices are designed to be worn by workers to test the air they're exposed to. They're small and lightweight, so they're non-obtrusive.
Active Air Sampling
Active air sampling requires a pump to collect air samples. Using a pump, it is possible to collect gases and vapors by pumping air through a tube containing a bed of a sorbent, or it is possible to collect particulates (aerosols) by pumping air through a filter housed in a cassette or size-selective sampler.
The flow rate is typically in the range of milliliters per minute for gases and vapors or liters per minute for aerosols. The flow rate needs to be set and verified using a calibrator designed for the purpose.
Some of the main advantages of active sampling include:
- Flow-rate variability: Because the pump has a variable speed, it is possible to collect the required sample over your preferred time period. However, optimal performance of size-selective aerosol samplers requires a fixed flow rate.
- Government validated methods: There is an enormous number of active sampling methods that have been validated and published by government agencies.
While this sample collection procedure forms the basis of the majority of methods, there are still some disadvantages of active air sampling for gases and vapors that may motivate some to choose passive sampling instead. Active air sampling is:
- Less user-friendly: Diffusive samplers are lighter and less obtrusive than active samplers, so wearers prefer them. Diffusive samplers do not require flow rate setting and measuring, so sample managers and hygienists prefer them.
- More costly: A bigger deterrent, in most cases, is the cost of the pump and calibrator. Of course, this cost is well worth the investment if you need to collect many samples, when the cost of the pump and calibrator will be spread between them.
Types of Active Air Sampling Equipment
Active air sampling involves three basic components:
- Collection media
These components work together to collect an air sample. There are a variety of active air testing systems available to choose from. For example, sorbent tubes containing collection materials like silica gel or activated charcoal are a popular choice for actively sampling gases and vapors. Preloaded air sampling cassettes are another convenient option for actively sampling aerosols.
When purchasing active air sampling equipment, you should always check to see which pumps a device is compatible with. In some cases, you may be able to use the same air sampling pump for various testing devices. You should also use calibration equipment to ensure your pumps are functioning correctly and are delivering accurate results.
Shop Online With Zefon International
No matter what testing method you employ, you want to use the very best sampling products available on the market. Make Zefon International your trusted source for quality air sampling products. We manufacture and sell a wide range of air sampling equipment to help you monitor indoor air quality. In addition to consistently high-quality products, you can also count on competitive pricing, fast delivery and dependable customer service when you shop for equipment with Zefon International.
If you have any questions about passive and active air sampling methods or the products available on our online store, contact us today.