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What Causes Emergency Department Crowding?
(Part 1 of 3)

May 19th, 2021
humanising healthcare

This article is the first in a three-part series which seeks to dissect the challenge of emergency department (ED) crowding; why it occurs, why it’s a problem and what we need to do to overcome it. The creation of this series is driven by the ever-pressing need to secure the sustainability of our EDs, which move closer and closer to breaking point with each day. This series has been curated with the support of senior Emergency Medicine Consultants from NHS England.

In this article, we will be drilling down into the details of how we define ED crowding and its causes.

ED crowding is an internationally recognised issue in healthcare, representing the most important problem facing emergency medicine. Crowding is one of the key factors contributing to the increasing pressure on our EDs. In England alone in 2018/19, for instance, 24.8 million accident and emergency department patients were treated, costing a total of £3.2bn, and of these patients, only 88% were treated within 4 hours. When compared with the 2011/12 figure of 98%, and the predicted 2025/26 figure of 79%, it is clear that supply is struggling to meet demand for emergency medicine [1, 2]. 

If you’ve visited a busy ED recently, you most likely don’t need these figures to tell you there’s a problem. They are visibly represented by queues of ambulances outside and bed trolleys in corridors, but the effects of crowding run deep into the heart of an ED, ultimately placing patient lives at elevated risk.

Initiatives have of course been launched to try and reduce crowding or its effects, but those efforts have been specific to sites, regions, or countries [3-7]. Until now, no internationally-validated solution has been proposed. More on that later in the series.

For now, let’s better understand the problem. 

So, What Is ED Crowding?

There’s currently no clear consensus or understanding on the correct tool or unit of measurement by which to define ED crowding [8]. One systematic review even identified 71 unique measures currently in use [9]. A good definition was proposed in 2011 which states that, fundamentally, ED crowding refers to ‘a situation where the demand for ED critical care exceeds available supply’ [10].

But what causes ED crowding, you ask?

Crowding in the ED can occur due to three different overarching factors; the volume of patients waiting to be seen (input), delays in assessments or treatment of patients in the ED (throughput), or exit blocks to patients leaving the ED (output) [11].

The input-throughput-output model, defined by Asplin et al in 2003, provides a clear framework to understand and target the root causes of crowding in specific areas of the ED. We break it down further below.


As the population grows, so too does the number of attendances to the ED. Input-related crowding has also been bolstered further by public health campaigns, focusing on time-critical conditions which have driven demand even higher [12]. The input causes of ED crowding are largely socio-economic.


Throughput causes of crowding are found once a patient has entered the ED. Facilities often aren’t equipped to deal with the demand due to inadequate staffing levels or physical layout of the ED, which causes constraints [12]. Reductions in patient flow can also be attributed to the time of day, with hospital occupancy often being highest in the evenings as patients wait to gain access to beds still occupied by other patients [12].

There has been some alleviation of staffing pressures through the rise of ambulatory care and seniority of ED physicians. However, this has also been coupled with increased investigation and time spent in the department [13].


Length of stay in the ED is increased by a shortage of inpatient beds and the reluctance of inpatient wards to admit patients [14]. As a result, there is reduced output from the ED, otherwise referred to as exit block, and this subsequently causes crowding.

In the UK, for example, issues arise with the declining bed base and shortcomings of many parts of the health system to adapt to increased demand. This is reflected in statistics which show the total number of NHS hospital beds in England have almost halved over the last 30 years, from roughly 299,000 in 1987/88, to 141,000 in 2018/19 [15]. These factors decrease output from the ED, resulting in crowding [10]. 

Despite the multifactorial causes of crowding, exit block is often considered one of, if not the, most significant contributing factors. If we are to overcome this challenge of ED crowding, output must be considered alongside input and throughout.

In conclusion, this is an extremely complex problem. We’re dealing with a huge number of potential triggers and root causes. While emergency care leaders can act on experience and gut feeling to combat against crowding, there is only so much you can do without a comprehensive, end-to-end overview of patient flow pinch points which vary from minute to minute, hour to hour and day to day.

The next article in this series will break down the impact of ED crowding on patient care to explain why this problem urgently requires our attention.

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1. Thorlby R, Gardner T, Turton C. NHS performance and waiting times. The Health Foundation website. 22 November 2019. Accessed April 30, 2021.

2. National cost collection 2018/19. NHS England website. Updated January 2020. Accessed April 30, 2021.

3. Fatovich D. Access block causes emergency department overcrowding and ambulance diversion in Perth, Western Australia. Emerg Med J. 2005;22(5):351-354.

4. Geelhoed G, Klerk N. Emergency department overcrowding, mortality and the 4‐hour rule in Western Australia. Med J Aust. 2012;196(2):122-126.

5. Khanna S, Boyle J, Good N, et al. Unravelling relationships: Hospital occupancy levels, discharge timing and emergency department access block. Emerg Med Australas. 2012;24(5):510-517.

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7. Crawford K, Morphet J, Jones T, et al. Initiatives to reduce overcrowding and access block in Australian emergency departments: A literature review. Collegian. 2014;21(4):359-366.

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9. Hwang U, McCarthy ML, Aronsky D, Asplin B, Crane PW, Craven CK, et al. Measures of crowding in the emergency department: a systematic review. Acad Emerg Med. 2011; 18(5):527–38. PMID: 21569171.

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11. Asplin BR, Magid DJ, Rhodes KV, Solberg LI, Lurie N, Camargo CA Jr. A conceptual model of emergency department crowding. Ann Emerg Med. 2003; 42(2):173–80. PMID: 12883504.

12. Boyle A, Higginson I. What should we do about crowding in emergency departments? British Journal of Hospital Medicine. 2018;79(9):1750-8460.

13. Wyatt et al, 2017 - Wyatt S, Child K, Hood A, Cooke M, Mohammed MA. Changes in admission thresholds in English emergency departments. Emerg Med J. 2017 Dec;34(12):773–779.

14. Boyle J, Jessup M, Crilly J, Green D, Lind J, Wallis M, et al. Predicting emergency department admissions. Emerg Med J. 2012;29(5):358-65.

15. Ewbank L, Thompson J, McKenna H, Anandaciva S. NHS hospital bed numbers: past, present, future. The King’s Fund website. March 2020. Accessed April 30, 2021.

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