Can you tell when a surface is clean? Let’s measure dirt!

Due to the current health circumstances that the world is experiencing, we are now more than aware about the importance that cleaning and disinfecting have in reducing the risk of disease transmission. But, is cleaning and disinfecting the same thing? Is it possible to measure the level of cleaning?

“80% of infections are spread through hand contact with hands or other objects”

For the first question, the answer is NO. Cleaning removes dirt, dust, crumbs, and germs from surfaces and objects; however, you only reduce the number of microbes on the surface. When disinfecting, we are actually killing those microbes, but in this case, we are not necessary removing dirt or germs. Another difference is the product type used: we normally use water and soap to clean, and chemical products to disinfect. In most cases, we will first clean a surface, and then disinfect it. Either cleaning, disinfecting or both, is known as sanitizing; which means to reduce the number of microbes to a safe level. This safe level is determined by public health standards1.

After sanitizing, can we measure the level of cleaning? YES. Let’s just be clear here, there is no standard method for measuring actual cleanliness of surfaces or the achievement of certain cleaning parameters (e.g. adequate contact time of disinfectant) or for defining the level of microbial contamination that correlates with good or poor environmental hygienic practices, like Guh and Carling state2. However, there are some tools that can help us with the task like direct observation, swab cultures, agar slide cultures, fluorescent gel or ATP system2 (Table 1).

Table 1: Methods to measure cleanliness and their characteristics (Modified from Guh and Carling, 2010).
Table 1: Methods to measure cleanliness and their characteristics (Modified from Guh and Carling, 2010).

Direct observation can be difficult since it required really well trained personal. Culture methodologies are the only way we have to identify pathogens, however, these are normally expensive and time-consuming. If we are looking for an easy method, which performance falls in between accuracy and velocity, we might use fluorescent markers or ATP bioluminescence. Florescent gel, powder or lotion is applied on high touch objects, marking these before being cleaned. After cleaning, we can check how much of the fluorescent product remains.

Adenosine Triphosphate (ATP) bioluminescence assay is a commonly used method for assessing environmental cleanliness on healthcare surfaces3. ATP is an indicator of organic material presence, rather than microbial contamination3. We will take a sample of the surface we want to analyse with a specialized swab. Then, we will introduce the swab into a test tube that contains luciferin-luciferase reagent, which reacts with any ATP residue present on a substrate, emitting light and measuring the presence of organic matter. This light emission is measured by a portable handheld luminometer that will show a number on the screen expressed in RLU/cm2 (Relative Light Units/ sampled area). There is a consensus on the cleanliness threshold being of 100 RLU4.

If you had an ATP luminometer apparatus at home, what objects do you think will be the dirtiest? The first thing I would analyse would be my cell phone or my computer keyboard, for sure. A study in 20095 showed that mobile phones are contaminated with various types of microorganisms and, when used in daily practice by health workers, they may be a source of nosocomial infections in hospitals. Koscova and colleagues6 found the common bacteria on skin, staphylococci, both on cell phones and computer keyboards, but also the potentially pathogenic species Staphylococcus aureus. Food-prepping areas are another candidate to be highly dirty. Cutting boards can produce cross-contamination with bacteria from animal sources like Salmonella7, and the waste reservoir of coffee machines harbours significant bacterial diversity, with Enterococcus sp. and Pseudomonas sp. being the main taxa8. And what about the bathroom? We would think that bathrooms are the worst places regarding to germs, however, according to NSF International9, the dirtiest place in the house is actually the kitchen. NSF swabbed for Coliform bacteria – a family of bacteria that includes Salmonella and E. coli and is an indicator of potential fecal contamination – and found that Coliform was found on:

  • More than 75% of dish sponges/rags
  • 45% of kitchen sinks
  • 32% of counter tops
  • 18% of cutting boards

This compares to the bathroom where areas with the most Coliform only included:

  • 27% of toothbrush holders
  • 9% of bathroom faucet handles

80% of infections are spread through hand contact with hands or other objects10, therefore, besides hand disinfection, surfaces also need to be included in our new disinfection routines in order to lower the risk of disease transmission.



  1. MedlinePlus. Cleaning, Disinfecting, and Sanitizing. Available at: (Accessed: 6th January 2021)
  2. Guh, A. & Carling, P. Options for Evaluating Environmental Cleaning | HAI | CDC. (2010). Available at: (Accessed: 5th January 2021)
  3. Sanna, T. et al. ATP bioluminescence assay for evaluating cleaning practices in operating theatres: Applicability and limitations. BMC Infect. Dis. 18, 583 (2018).
  4. Whiteley, G. S., Glasbey, T. O. & Fahey, P. P. A suggested sampling algorithm for use with ATP testing in cleanliness measurement. Infect. Dis. Heal. 21, 169–175 (2016).
  5. Ulger, F. et al. Are we aware how contaminated our mobile phones with nosocomial pathogens? Ann. Clin. Microbiol. Antimicrob. 8, 7 (2009).
  6. Koscova, J., Hurnikova, Z. & Pistl, J. Degree of bacterial contamination of mobile phone and computer keyboard surfaces and efficacy of disinfection with chlorhexidine digluconate and triclosan to its reduction. Int. J. Environ. Res. Public Health 15, (2018).
  7. Park, P. K. & Cliver, D. O. Cutting boards in Salmonella cross-contamination. J. Food Prot. 59, 1049–1054 (1996).
  8. Vilanova, C., Iglesias, A. & Porcar, M. The coffee-machine bacteriome: Biodiversity and colonisation of the wasted coffee tray leach. Sci. Rep. 5, 17163 (2015).
  9. International, N. Detailed Swab Analysis. (2011).
  10. Al-Ghamdi, A. K. et al. Bacterial contamination of computer keyboards and mice, elevator buttons and shopping carts. African J. Microbiol. Res. 5, 3998–4003 (2011).